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: Nkbuild test robot <fengguang.wu@intel.com>
Signed-off-by: NGregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: NLinus Walleij <linus.walleij@linaro.org>

drivers/gpio/Kconfig

@@ -329,6 +329,7 @@ config GPIO_MVEBU
 	depends on PLAT_ORION || ARCH_MVEBU
 	depends on OF_GPIO
 	select GENERIC_IRQ_CHIP
+	select REGMAP_MMIO
 
 config GPIO_MXC
 	def_bool y