rt2x00: Add rt2x00soc bus module

Add new library module for SoC drivers.
This is needed to fully support the platform
driver part of rt2800pci.

Based on original patch from Felix.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>