rt2x00: Introduce SoC interface type.

Introduce the SoC interface type to detect SoC devices, instead of having
them mimic being PCI devices.
This allows for easier detection of SoC devices.

Signed-off-by: Gertjan van Wingerde <gwingerde@gmail.com>
Acked-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>