staging: mt7621-pci: MIPS/ralink: add MT7621 pcie driver

NeilBrown: forward port and hack to work on GNUBEE1

Signed-off-by: John Crispin <blogic@openwrt.org>
Signed-off-by: NeilBrown <neil@brown.name>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>