pinctrl: mvebu: add common mpp reg helper to mvebu pinctrl include

This adds some defines and helper functions for the common mpp reg
layout to mvebu pinctrl include.

Signed-off-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Tested-by: Andrew Lunn <andrew@lunn.ch>