pinctrl: mvebu: armada-39x: add missing PCIe functions

The latest version of the Armada 39x datasheet documents several new
PCIe related functions on various MPP pins. This commit adds the
description of these new functions to the Armada 39x 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>