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>