ath79: add support for Senao Engenius ESR900

ath79: add support for Senao Engenius ESR900

FCC ID: A8J-ESR900

Engenius ESR900 is an indoor wireless router with
a gigabit ethernet switch, dual-band wireless,
internal antenna plates, and a USB 2.0 port

**Specification:**

  - QCA9558 SOC		2.4 GHz, 3x3
  - AR9580 WLAN		PCIe on board, 5 GHz, 3x3
  - AR8327N SW		4 ports LAN, 1 port WAN
  - 40 MHz clock
  - 16 MB FLASH		MX25L12845EMI-10G
  - 2x 64 MB RAM
  - UART at J1		populated, RX grounded
  - 6 internal antenna plates (omni-directional)
  - 5 LEDs, 1 button (power, 2G, 5G, WAN, WPS) (reset)

**MAC addresses:**

  Base MAC address labeled as "MAC ADDRESS"
  MAC "wanaddr" is not similar to "ethaddr"

  eth0 *:06 MAC u-boot-env ethaddr
  phy0 *:06 MAC u-boot-env ethaddr
  phy1 *:07 --- u-boot-env ethaddr +1
  WAN  *:6E:81  u-boot-env wanaddr

**Serial Access:**

  RX 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 RX pin

**Installation:**

  Method 1: Firmware upgrade page

    OEM webpage at 192.168.0.1
    username and password "admin"
    Navigate to Settings (gear icon) --> Tools --> Firmware
    select the factory.bin image
    confirm and wait 3 minutes

  Method 2: TFTP recovery

    Follow TFTP instructions using initramfs.bin
    use sysupgrade.bin to flash using openwrt web interface

**Return to OEM:**

  MTD partitions should be backed up before flashing
  using TFTP to boot openwrt without overwriting flash

  Alternatively, it is possible to edit OEM firmware images
  to flash MTD partitions in openwrt to restore OEM firmware
  by removing the OEM header and writing the rest to "firmware"

**TFTP recovery:**

  Requires serial console, reset button does nothing at boot

  rename initramfs.bin to 'uImageESR900'
  make available on TFTP server at 192.168.99.8
  power board, interrupt boot by pressing '4' rapidly
  execute tftpboot and bootm

**Note on ETH switch registers**

  Registers must be written to the ethernet switch
  in order to set up the switch's MAC interface.
  U-boot can write the registers on it's own
  which is needed, for example, in a TFTP transfer.

  The register bits from OEM for the AR8327 switch
  can be read from interrupted boot (tftpboot, bootm)
  by adding print lines in the switch driver ar8327.c
  before 'qca,ar8327-initvals' is parsed from DTS and written.
  for example:

    pr_info("0x04 %08x\n", ar8xxx_read(priv, AR8327_REG_PAD0_MODE));

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