devicetree: bindings: add Device Tree bindings for Armada 39x pin-muxing controller

This commit adds the Device Tree binding documentation to describe the
pin-muxing controller of the Marvell Armada 39x processors. Two
variants are supported for the moment: the 88F6920 (Armada 390) and
88F6928 (Armada 398).

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