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