rt2x00: create a new module for rt2800 MMIO code

Create a new module for common code which can be used
for rt2800 device with memory mapped I/O. It is an empty
module for now, but it will be populated by subsequent
patches.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>