GPIO: MIPS: lantiq: move gpio-stp and gpio-ebu to the subsystem folder

Move the 2 drivers from arch/mips/lantiq/xway/ to the subsystem and make them
buildable.

The following 2 patches will convert the drivers to OF.

Signed-off-by: John Crispin <blogic@openwrt.org>
Cc: linux-kernel@vger.kernel.org
Cc: linux-mips@linux-mips.org
Acked-by: Grant Likely <grant.likely@secretlab.ca>
Patchwork: https://patchwork.linux-mips.org/patch/3838/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>