pinctrl: armada-37xx: Add gpio support

GPIO management is pretty simple and is part of the same IP than the pin
controller for the Armada 37xx SoCs.  This patch adds the GPIO support to
the pinctrl-armada-37xx.c file, it also allows sharing common functions
between the gpiolib and the pinctrl drivers.

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