ar71xx: add support for ALFA Network AP91-5G

ar71xx: add support for ALFA Network AP91-5G

ALFA Network AP91-5G is a 5 GHz outdoor AP/CPE board, based on Atheros
AR7240 + AR9280.

Specification:

- 400/400/200 MHz (CPU/DDR/AHB)
- 32 MB of RAM (DDR1)
- 8 MB of FLASH (SPI NOR)
- 1x 10/100 Mbps Ethernet, with passive PoE support (24 V)
- 1T1R 5 GHz (AR9280), with ext. PA (SE5004L) and LNA, up to 27 dBm
- 6x LED (5 are driven by GPIO)
- 1x button (reset)
- external h/w watchdog (EM6324QYSP5B, disabled and not used)
- header for optional 802.3at/af PoE module
- DC jack for main power input (optional, not installed by default)
- UART and LEDs headers on PCB

Flash instruction:

Use "factory" image in vendor GUI (in case of problems, make sure your
board has up to date firmware). Alternatively, TFTP in U-Boot can be
used: select option "2. Load system code then write to Flash via TFTP"
during early boot and use "sysupgrade" image.

Signed-off-by: Piotr Dymacz <pepe2k@gmail.com>