GPIO: MIPS: lantiq: convert gpio-stp-xway to OF

GPIO: MIPS: lantiq: convert gpio-stp-xway to OF

Implements OF support and add code to load custom properties from the DT.

The Serial To Parallel (STP) is found on MIPS based Lantiq socs. It is a
peripheral controller used to drive external shift register cascades. At most
3 groups of 8 bits can be driven. The hardware is able to allow the DSL modem
to drive the 2 LSBs of the cascade automatically. Newer socs are also able to
automatically drive some pins via the internal PHYs. The driver currently only
supports output functionality. Patches for the input feature found on newer
generations of the soc will be provided in a later series.

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/3839/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>