gpio: brcmstb: Allow building driver for BMIPS_GENERIC

BMIPS_GENERIC (arch/mips/bmips) is the Kconfig symbol associated with
Broadcom MIPS-based STB chips. Since this driver is perfectly usable on
these platforms as well, allow using it.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Acked-by: Gregory Fong <gregory.0xf0@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>