[MIPS] make fallback gpio.h gpiolib-friendly

If gpiolib was selected, asm-generic/gpio.h provides some prototypes
for gpio API and implementation helpers.  With this patch, platform
code can implement its GPIO API using gpiolib without custom gpio.h
file.

Signed-off-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>