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

Add device tree bindings for the gpio controller on
the Microchip PolarFire SoC.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Acked-by: Palmer Dabbelt <palmer@rivosinc.com>
Acked-by: Bartosz Golaszewski <brgl@bgdev.pl>
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>