For consistency with the datasheet, this commit renames the VDD
function of the MPP4 pin.

While this changes the DT compatibility, it is not considered to be a
problem since this pin is unlikely to be used for anything but
debugging purposes.
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>