dt-bindings: marvell: Add documentation for the North Bridge PM on Armada 37xx

Extend the documentation of the Armada 37xx SoC with the the North
Bridge Power Management component.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>