ramips: re-implement PCI mux

Parallel flash access on RT3662/RT3883 based devices
is broken without that. It was fixed already some time
ago, but that change seems lost somewhere.

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

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@37708 3c298f89-4303-0410-b956-a3cf2f4a3e73