pinctrl: mvebu: armada-xp: provide generic mpp callbacks

We want to get rid of passing register addresses to common pinctrl
driver, so provide set/get callbacks that use generic mpp pins helper
and will be used later.

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