ath79: add support for Senao WatchGuard AP200

ath79: add support for Senao WatchGuard AP200

FCC ID: U2M-CAP4200AG

WatchGuard AP200 is an indoor wireless access point with
1 Gb ethernet port, dual-band wireless,
internal antenna plates, and 802.3at PoE+

this board is a Senao device:
the hardware is equivalent to EnGenius EAP600
the software is modified Senao SDK which is based on openwrt and uboot
including image checksum verification at boot time,
and a failsafe image that boots if checksum fails

**Specification:**

  - AR9344 SOC		MIPS 74kc, 2.4 GHz WMAC, 2x2
  - AR9382 WLAN		PCI card 168c:0030, 5 GHz, 2x2, 26dBm
  - AR8035-A EPHY	RGMII GbE with PoE+ IN
  - 25 MHz clock
  - 16 MB FLASH		mx25l12805d
  - 2x 64 MB RAM
  - UART console        J11, populated
  - GPIO watchdog       GPIO 16, 20 sec toggle
  - 4 antennas          5 dBi, internal omni-directional plates
  - 5 LEDs              power, eth0 link/data, 2G, 5G
  - 1 button            reset

**MAC addresses:**

  Label has no MAC
  Only one Vendor MAC address in flash at art 0x0

  eth0 ---- *:be art 0x0 -2
  phy1 ---- *:bf art 0x0 -1
  phy0 ---- *:be art 0x0 -2

**Installation:**

  Method 1: OEM webpage

    use OEM webpage for firmware upgrade to upload factory.bin

  Method 2: root shell

    It may be necessary to use a Watchguard router to flash the image to the AP
    and / or to downgrade the software on the AP to access SSH
    For some Watchguard devices, serial console over UART is disabled.

  NOTE: DHCP is not enabled by default after flashing

**TFTP recovery:**

  reset button has no function at boot time
  only possible with modified uboot environment,
  (see commit message for Watchguard AP300)

**Return to OEM:**

  user should make backup of MTD partitions
  and write the backups back to mtd devices
  in order to revert to OEM reliably

  It may be possible to use sysupgrade
  with an OEM image as well...
  (not tested)

**OEM upgrade info:**

  The OEM upgrade script is at /etc/fwupgrade.sh

  OKLI kernel loader is required because the OEM software
  expects the kernel to be no greater than 1536k
  and the factory.bin upgrade procedure would otherwise
  overwrite part of the kernel when writing rootfs.

**Note on eth0 PLL-data:**

  The default Ethernet Configuration register values will not work
  because of the external AR8035 switch between
  the SOC and the ethernet port.

  For AR934x series, the PLL registers for eth0
  can be see in the DTSI as 0x2c.
  Therefore the PLL registers can be read from uboot
  for each link speed after attempting tftpboot
  or another network action using that link speed
  with `md 0x1805002c 1`.

  The clock delay required for RGMII can be applied
  at the PHY side, using the at803x driver `phy-mode`.
  Therefore the PLL registers for GMAC0
  do not need the bits for delay on the MAC side.
  This is possible due to fixes in at803x driver
  since Linux 5.1 and 5.3

**Note on WatchGuard Magic string:**

  The OEM upgrade script is a modified version of
  the generic Senao sysupgrade script
  which is used on EnGenius devices.

  On WatchGuard boards produced by Senao,
  images are verified using a md5sum checksum of
  the upgrade image concatenated with a magic string.
  this checksum is then appended to the end of the final image.

  This variable does not apply to all the senao devices
  so set to null string as default

Tested-by: Steve Wheeler <stephenw10@gmail.com>
Tested-by: John Delaney <johnd@ankco.net>
Signed-off-by: Michael Pratt <mcpratt@pm.me>