pinctrl: mvebu: kirkwood pinctrl driver

This patch adds a SoC specific pinctrl driver for Marvell Kirkwood SoCs
plus DT binding documentation. This driver will use the mvebu pinctrl
driver core.

Signed-off-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
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>
Tested-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>