dt-bindings: reset: Add binding constants for Actions S500 RMU

Add device tree binding constants for Actions Semi S500 SoC Reset
Management Unit (RMU).

Signed-off-by: Cristian Ciocaltea <cristian.ciocaltea@gmail.com>
Acked-by: Philipp Zabel <p.zabel@pengutronix.de>
Link: https://lore.kernel.org/r/daf615160b3be9f38dcf7926cc82128c9c2d73e3.1593788312.git.cristian.ciocaltea@gmail.com
Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>