ipq40xx: add support for OpenMesh A62

ipq40xx: add support for OpenMesh A62

* QCA IPQ4019
* 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=20,variant=OM-A62
* 2T2R 5 GHz (channel 36-64)
  - QCA9888 hw2.0 (PCI)
  - requires special BDF in QCA9888/hw2.0/board-2.bin
    bus=pci,bmi-chip-id=0,bmi-board-id=16,variant=OM-A62
* 2T2R 5 GHz (channel 100-165)
  - QCA4019 hw1.0 (SoC)
  - requires special BDF in QCA4019/hw1.0/board-2.bin with
    bus=ahb,bmi-chip-id=0,bmi-board-id=21,variant=OM-A62
* 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
  - phy@mdio3:
    + Label: Ethernet 1
    + gmac0 (ethaddr) in original firmware
    + 802.3at POE+
  - phy@mdio4:
    + Label: Ethernet 2
    + gmac1 (eth1addr) in original firmware
    + 18-24V passive POE (mode B)
* powered only via POE

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.

The initramfs image can be started using

  setenv bootargs 'loglevel=8 earlycon=msm_serial_dm,0x78af000 console=ttyMSM0,115200 mtdparts=spi0.0:256k(0:SBL1),128k(0:MIBIB),384k(0:QSEE),64k(0:CDT),64k(0:DDRPARAMS),64k(0:APPSBLENV),512k(0:APPSBL),64k(0:ART),64k(0:custom),64k(0:KEYS),15552k(inactive),15552k(inactive2)'
  tftpboot 0x84000000 openwrt-ipq40xx-openmesh_a62-initramfs-fit-uImage.itb
  set fdt_high 0x85000000
  bootm 0x84000000

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