ar71xx: rb91x-nand: rewrite to use GPIO API

Rewrite tha rb91x-nand driver to use GPIO API to
modify the NAND control lines.

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

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@39700 3c298f89-4303-0410-b956-a3cf2f4a3e73