dt-bindings: rtc: add bindings for microchip mpfs rtc

Add device tree bindings for the real time clock on
the Microchip PolarFire SoC.

Signed-off-by: Daire McNamara <daire.mcnamara@microchip.com>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Acked-by: Palmer Dabbelt <palmer@rivosinc.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>