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

Make it possible to build GPIO on r8a7791.

Signed-off-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>
diff --git a/arch/arm/mach-shmobile/Kconfig b/arch/arm/mach-shmobile/Kconfig
index ff7c4ce..4e1cc76 100644
--- a/arch/arm/mach-shmobile/Kconfig
+++ b/arch/arm/mach-shmobile/Kconfig
@@ -97,6 +97,7 @@
 
 config ARCH_R8A7791
 	bool "R-Car M2 (R8A77910)"
+	select ARCH_WANT_OPTIONAL_GPIOLIB
 	select ARM_GIC
 	select CPU_V7
 	select SH_CLK_CPG