pinctrl: mvebu: armada-xp: add nand rb function

The latest version of the Armada XP datasheet documents a new
NAND-related MPP function on MPP48, for which this commit adds
support.

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