ar71xx: add Abicom SC450 support

Support Abicom International Scorpion SC450 Board

QCA9550 700MHz Extended Temperature Range
256MB DDR2
256MB NAND Flash
16MB NOR Flash
10/100/1000 Ethernet
15W Max, 3x3 Mode, Full Power TX Power
802.3-at POE+ & DC Input options
100x 85mm
USB
PCIe
SFP
GPIO/LED

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