arm: kirkwood: Add support for ZyXEL NSA325 board

ZyXEL NSA325 specifications:

Marvell Kirkwood 88F6282 SoC
1.6 GHz CPU
1x GBE LAN port (Marvell MV88E1318)
512 MB RAM
128 MB Eon NAND, SLC
I2C
1x USB 3.0 (on PCIe bus)
2x USB 2.0
2x SATA (hot swap slots)
Serial console

Signed-off-by: Tony Dinh <mibodhi@gmail.com>
Reviewed-by: Stefan Roese <sr@denx.de>