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: NFelix Fietkau <nbd@openwrt.org>
Signed-off-by: NIvo van Doorn <IvDoorn@gmail.com>
Signed-off-by: NJohn W. Linville <linville@tuxdriver.com>