net:rfkill: add a gpio setup function into GPIO rfkill

Add a gpio setup function which gives a chance to set up
platform specific configuration such as pin multiplexing,
input/output direction at the runtime or booting time.

Signed-off-by: Sangwook Lee <sangwook.lee@linaro.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>