ath79: add support for Senao Engenius ECB1200

ath79: add support for Senao Engenius ECB1200

FCC ID: A8J-ECB1200

Engenius ECB1200 is an indoor wireless access point with a GbE port,
2.4 GHz and 5 GHz wireless, external antennas, and 802.3af PoE.

**Specification:**

  - QCA9557 SOC			MIPS, 2.4 GHz (2x2)
  - QCA9882 WLAN		PCIe card, 5 GHz (2x2)
  - AR8035-A switch		RGMII, GbE with 802.3af PoE, 25 MHz clock
  - 40 MHz reference clock
  - 16 MB FLASH			25L12845EMI-10G
  - 2x 64 MB RAM		1538ZFZ V59C1512164QEJ25
  - UART at JP1			(unpopulated, RX shorted to ground)
  - 4 external antennas
  - 4 LEDs, 1 button		(power, eth, wifi2g, wifi5g) (reset)

**MAC addresses:**

  MAC Addresses are labeled as ETH and 5GHZ
  U-boot environment has the vendor MAC addresses
  MAC addresses in ART do not match vendor

  eth0	ETH	*:5c	u-boot-env ethaddr
  phy0	5GHZ	*:5d	u-boot-env athaddr
  ----	----	????	art 0x0/0x6

**Installation:**

  Method 1: Firmware upgrade page:

  OEM webpage at 192.168.1.1
  username and password "admin"
  Navigate to "Firmware" page from left pane
  Click Browse and select the factory.bin image
  Upload and verify checksum
  Click Continue to confirm and wait 3 minutes

  Method 2: Serial to load Failsafe webpage:

  After connecting to serial console and rebooting...
  Interrupt uboot with any key pressed rapidly

  (see TFTP recovery)
  perform a sysupgrade

**Serial Access:**

  the RX line on the board for UART is shorted to ground by resistor R176
  therefore it must be removed to use the console
  but it is not necessary to remove to view boot log
  optionally, R175 can be replaced with a solder bridge short

  the resistors R175 and R176 are next to the UART pinout at JP1

**Return to OEM:**

  If you have a serial cable, see Serial Failsafe instructions

  Unlike most Engenius boards, this does not have a 'failsafe' image
  the only way to return to OEM is TFTP or serial access to u-boot

**TFTP recovery:**

  Unlike most Engenius boards, TFTP is reliable here

  rename initramfs-kernel.bin to 'ap.bin'
  make the file available on a TFTP server at 192.168.1.10
  power board while holding or pressing reset button repeatedly

  or with serial access:
  run `tftpboot` or `run factory_boot` with initramfs-kernel.bin
  then `bootm` with the load address

**Format of OEM firmware image:**

  The OEM software of ECB1200 is a heavily modified version
  of Openwrt Altitude Adjustment 12.09.

  This Engenius board, like ECB1750, uses a proprietary header
  with a unique Product ID. The header for factory.bin is
  generated by the mksenaofw program included in openwrt.

**Note on PLL-data cells:**

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

  For QCA955x series, the PLL registers for eth0 and eth1
  can be see in the DTSI as 0x28 and 0x48 respectively.
  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 0x18050028 1` and `md 0x18050048 1`.

  However the registers that u-boot sets are not ideal and sometimes wrong...
  the at803x driver supports setting the RGMII clock/data delay on the PHY side.
  This way the pll-data register only needs to handle invert and phase.

  for this board clock invert is needed on the MAC side
  all link speeds functional

Signed-off-by: Michael Pratt <mcpratt@pm.me>