ath79: Add support for Plasma Cloud PA300

ath79: Add support for Plasma Cloud PA300

Device specifications:

* Qualcomm/Atheros QCA9533 v2
* 650/600/217 MHz (CPU/DDR/AHB)
* 64 MB of RAM
* 16 MB of SPI NOR flash (mx25l12805d)
  - 2x 7 MB available; but one of the 7 MB regions is the recovery image
* 2x 10/100 Mbps Ethernet
* 2T2R 2.4 GHz Wi-Fi
* multi-color LED (controlled via red/green/blue GPIOs)
* 1x GPIO-button (reset)
* external h/w watchdog (enabled by default)
* TTL pins are on board (arrow points to VCC, then follows: GND, TX, RX)
* 2x fast ethernet
  - eth0
    + Label: Ethernet 1
    + 24V passive POE (mode B)
    + used as WAN interface
  - eth1
    + Label: Ethernet 2
    + 802.3af POE
    + builtin switch port 2
    + used as LAN interface
* 12-24V 1A DC
* internal antennas

Flashing instructions:

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@narfation.org>