pinctrl: mvebu: armada-xp: remove non-existing VDD cpu_pd functions

The latest version of the Armada XP datasheet no longer documents the
VDD cpu_pd functions, which might indicate they are not working and/or
not supported. This commit ensures the pinctrl driver matches the
datasheet.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: <stable@vger.kernel.org> # v3.7+
Fixes: 463e270f ("pinctrl: mvebu: add pinctrl driver for Armada XP")
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>