pinctrl: armada-37xx: Add pin controller support for Armada 37xx

pinctrl: armada-37xx: Add pin controller support for Armada 37xx

The Armada 37xx SoC come with 2 pin controllers: one on the south
bridge (managing 28 pins) and one on the north bridge (managing 36 pins).

At the hardware level the controller configure the pins by group and not
pin by pin. This constraint is reflected in the design of the driver:
only the group related functions are implemented.

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