ARM: brcmstb: Select ARCH_WANT_OPTIONAL_GPIOLIB

Select ARCH_WANT_OPTIONAL_GPIOLIB from BRCMSTB to allow GPIOLIB and
GPIO_BRCMSTB to be enabled.

Signed-off-by: Gregory Fong <gregory.0xf0@gmail.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>