gpio: mvebu: switch to regmap for register access

In order to be able to use this driver with the Armada 7K/8K SoCs, we
need to use the regmap to access the registers. Indeed for these new SoCs,
the gpio node will be part of a syscon.

[gregory.clement@free-electrons.com:
 - fixed merge conflcit from 4.10 to 4.12-rc1
 - added a commit log]

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>