Documentation: bindings: add reset bindings docs for Marvell Berlin SoCs

Add the reset binding documentation to the SoC binding documentation as
the reset driver in Marvell Berlin SoC is part of the chip/system
control registers. This patch adds the required properties to configure
the reset controller.

Signed-off-by: Antoine Ténart <antoine.tenart@free-electrons.com>
Acked-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>