dt-binding: ARM: add pinctrl binding docs for Marvell Berlin2 SoCs

Add pin control binding documentation to the SoC binding documentaion
as pinctrl is part of chip/system control registers. The documentation
also explains how to configure this group based controller.

Signed-off-by: Antoine Tenart <antoine.tenart@free-electrons.com>
Signed-off-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>