ARM: kirkwood: Add support for NETGEAR ReadyNAS Duo v2 using DT

Add support for NETGEAR ReadyNAS Duo v2 (Hardware specs available
here: http://natisbad.org/NAS/

). Almost everything is supported via
provided .dts. A board-readynas.c file is nonetheless required for
device not only converted to DT (Gbit controller).

Signed-off-by: Arnaud Ebalard <arno@natisbad.org>
Tested-By: Arnaud Ebalard <arno@natisbad.org>
Acked-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>