MIPS: BMIPS: Enable ARCH_WANT_OPTIONAL_GPIOLIB

Allow BMIPS_GENERIC supported platforms to build GPIO controller
drivers.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Dragan Stancevic <dragan.stancevic@gmail.com>
Cc: cernekee@gmail.com
Cc: jaedon.shin@gmail.com
Cc: gregory.0xf0@gmail.com
Cc: Florian Fainelli <f.fainelli@gmail.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/12019/


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