ARM: shmobile: Select GPIO in case of the r8a7791 SoC

Make it possible to build GPIO on r8a7791.
Signed-off-by: NMagnus Damm <damm@opensource.se>
Signed-off-by: NSimon Horman <horms+renesas@verge.net.au>

arch/arm/mach-shmobile/Kconfig

@@ -97,6 +97,7 @@ config ARCH_R8A7790
 
 config ARCH_R8A7791
 	bool "R-Car M2 (R8A77910)"
+	select ARCH_WANT_OPTIONAL_GPIOLIB
 	select ARM_GIC
 	select CPU_V7
 	select SH_CLK_CPG