gpio: mvebu: Add support for the Armada 7K/8K SoCs

The Armada 7K and 8K SoCs use the same gpio controller as most of the
other mvebu SoCs. However, the main difference is that the GPIO
controller is part of a bigger system controller, and a syscon is used to
control the overall system controller. Therefore, the driver needs to be
adjusted to retrieve the regmap of the syscon to access registers, and
account for the fact that registers are located at a certain offset
within the regmap.

This commit add the support of the syscon and introduce a new variant for
this case.

It was based on the preliminary work of Thomas Petazzoni.
Tested-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>