pinctrl: mvebu: armada-xp: add dram functions

The latest Armada XP datasheet documents several new DRAM related
functions on various MPPs. This commit adds the description of these
new functions in the Armada XP pinctrl driver and its DT binding
documentation.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>