ARM: shmobile: Select GPIO in case of the r7s72100 SoC

The r7s72100 contains GPIO controllers so make sure the GPIO
subsystem can be built by selecting ARCH_WANT_OPTIONAL_GPIOLIB.

Signed-off-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>