gpio: mvebu: Select REGMAP_MMIO now that regmap is used

Since the commit "gpio: mvebu: switch to regmap for register access" the
driver use the regmap. Explicitly select the REGMAP_MMIO symbol to fix
build error.

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
diff --git a/drivers/gpio/Kconfig b/drivers/gpio/Kconfig
index f5e1b3d..89e805f 100644
--- a/drivers/gpio/Kconfig
+++ b/drivers/gpio/Kconfig
@@ -329,6 +329,7 @@
 	depends on PLAT_ORION || ARCH_MVEBU
 	depends on OF_GPIO
 	select GENERIC_IRQ_CHIP
+	select REGMAP_MMIO
 
 config GPIO_MXC
 	def_bool y