pinctrl: armada-37xx: Add gpio support

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 gpio and the pinctrl drivers.

Ported to U-Boot based on the Linux version by Stefan Roese.

Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Gregory CLEMENT <gregory.clement@free-electrons.com>
Cc: Konstantin Porotchkin <kostap@marvell.com>
Cc: Nadav Haklai <nadavh@marvell.com>