pinctrl: mvebu: add pinctrl driver for Marvell Armada 39x

This commit adds a new pinctrl driver for the Marvell Armada 39x
family of processors, which hooks into the existing infrastructure to
support pin-muxing on Marvell EBU processors. Two variants of the
Armada 39x are supported: 88F6920 (Armada 390) and 88F6928 (Armada
398), which have a few differences in the available functions for
certain pins.

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