dt-bindings: arm: add DT binding for Marvell AP806 system controller

This commit adds the Device Tree binding documentation for the system
controller found in Marvell AP806 HW block, which is one of the core
HW blocks of the 64-bits Marvell Armada 7K/8K family.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>