ar71xx: add Abicom SC300M support

Support Abicom International Scorpion SC300M Module

QCA9550 700MHz Extended Temperature Range
256MB DDR2
256MB Nand Flash
16MB SPI Flash
802.11a/na/b/g/ng
GPIO x 22, USB x2, PCIe x2
10/100/1000 Ethernet
23dBm, Optional Onboard Antenna
DC Input, 8-20V DC
85 x 55 mm

Signed-off-by: Conor O'Gorman <i@conorogorman.net>