ARM: mvebu: Add Netgear ReadyNAS 2120 board

All hardware parts of the (mv78230 Armada XP based) NETGEAR ReadyNAS
2120 are supported by mainline kernel (USB 3.0 and eSATA rear ports,
USB 2.0 front port, Gigabit controller and PHYs for the two rear ports,
serial port, LEDs, Buttons, 88SE9170 SATA controllers, three G762 fan
controllers, G751 temperature sensor) except for:

 - the Intersil ISL12057 I2C RTC Chip,
 - the Armada NAND controller.

Support for both of those is currently work in progress and does not
prevent boot.

Signed-off-by: Arnaud Ebalard <arno@natisbad.org>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>