pinctrl: mvebu: remove MPP_REG_CTRL macro

Now that each per-SoC pinctrl driver must implement its own get/set
functions, there is no point in keeping the MPP_REG_CTRL macro, whose
purpose was to let the core pinctrl mvebu driver use default get/set
functions. While at it also update the comment about mvebu_mpp_ctrl.

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