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