ar71xx: add NAND driver for the Mikrotik RB91x boards

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@39097 3c298f89-4303-0410-b956-a3cf2f4a3e73