dt-bindings: pwm: add microchip corepwm binding

Add device tree bindings for the Microchip fpga fabric based "core" PWM
controller.

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: Uwe Kleine-K=F6nig <u.kleine-koenig@pengutronix.de>
Acked-by: Thierry Reding <thierry.reding@gmail.com>
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>