ipq806x: add support for OpenMesh A42

ipq806x: add support for OpenMesh A42

* QCA IPQ401x
* 256 MB of RAM
* 32 MB of SPI NOR flash (s25fl256s1)
  - 2x 15 MB available; but one of the 15 MB regions is the recovery image
* 2T2R 2.4 GHz
  - QCA4019 hw1.0 (SoC)
  - requires special BDF in QCA4019/hw1.0/board-2.bin with
    bus=ahb,bmi-chip-id=0,bmi-board-id=16,variant=OM-A42
* 2T2R 5 GHz
  - QCA4019 hw1.0 (SoC)
  - requires special BDF in QCA4019/hw1.0/board-2.bin with
    bus=ahb,bmi-chip-id=0,bmi-board-id=17,variant=OM-A42
* multi-color LED (controlled via red/green/blue GPIOs)
* 1x button (reset; kmod-input-gpio-keys compatible)
* external watchdog
  - triggered GPIO
* 1x USB (xHCI)
* TTL pins are on board (arrow points to VCC, then follows: GND, TX, RX)
* 2x gigabit ethernet
* powered only via POE
  - 802.3af POE on Ethernet 1
  - 18-24v passive POE (mode B) on Ethernet 2

The tool ap51-flash (https://github.com/ap51-flash/ap51-flash) should be
used to transfer the factory image to the u-boot when the device boots up.

Signed-off-by: Sven Eckelmann <sven.eckelmann@open-mesh.com>