dt-bindings: reset: Add binding constants for Actions Semi S900 RMU

Add device tree binding constants for Actions Semi S900 SoC Reset
Management Unit (RMU).

Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>