Add device tree bindings for the MSS system controller mailbox on
the Microchip PolarFire SoC.
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>