pinctrl: mvebu: armada-38x: add nand functions

The latest version of the Armada 38x datasheet documents several new
NAND related functions on various MPP pins. This commit adds the
description of these new functions to the Armada 38x pinctrl driver as
well as to its DT binding documentation.

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