MIPS: Lantiq: Add more gpio drivers

The XWAY family allows to extend the number of gpios by using shift
registers or latches. This patch adds the 2 drivers needed for this. The
extended gpios are output only.

[ralf@linux-mips.org: Fixed ltq_stp_probe section() attributes.]

Signed-off-by: John Crispin <blogic@openwrt.org>
Signed-off-by: Ralph Hempel <ralph.hempel@lantiq.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/2258/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>