pinctrl: mvebu: add pinctrl driver for Armada XP

This pinctrl driver is not a full-blown pinctrl driver from scratch:
it relies on the common pinctrl-mvebu driver, which is used for all
Marvell EBU SoCs.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Tested-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: Stephen Warren <swarren@wwwdotorg.org>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>