gpio: introduce gpio-mvebu driver for Marvell SoCs

gpio: introduce gpio-mvebu driver for Marvell SoCs

This driver aims at replacing the arch/arm/plat-orion/gpio.c driver,
and is designed to be compatible with all Marvell EBU SoCs: Orion,
Kirkwood, Dove, Armada 370/XP and Discovery.

It has been successfully tested on Dove and Armada XP at the moment.

Compared to the plat-orion driver, this new driver has the following
added benefits:

 *) Support for Armada 370 and Armada XP
 *) It is integrated with the mvebu pinctrl driver so that GPIO pins
    are properly muxed, and the GPIO driver knows which GPIO pins are
    output-only or input-only.
 *) Properly placed in drivers/gpio
 *) More extensible mechanism to support platform differences. The
    plat-orion driver uses a simple mask-offset DT property, which
    works fine for Discovery MV78200 but not for Armada XP. The new
    driver uses different compatible strings to identify the different
    variants of the GPIO controllers.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Grant Likely <grant.likely@secretlab.ca>
Cc: Linus Walleij <linus.walleij@stericsson.com>
Cc: Andrew Lunn <andrew@lunn.ch>
Cc: Jason Cooper <jason@lakedaemon.net>
Cc: Gregory Clement <gregory.clement@free-electrons.com>
Tested-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Tested-by: Andrew Lunn <andrew@lunn.ch>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>