gpio: brcmstb: Set endian flags for big-endian MIPS

Broadcom MIPS-based STB chips endianness is configured by boot strap,
which also reverses all bus endianness (i.e., big-endian CPU + big
endian bus ==> native endian I/O).

Other architectures (e.g., ARM) either do not support big endian, or
else leave I/O in little endian mode.

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>