dt-bindings: pwm: Add bindings for OpenCores PWM Controller

Add bindings for OpenCores PWM Controller.

Signed-off-by: William Qiu <william.qiu@starfivetech.com>
Reviewed-by: Hal Feng <hal.feng@starfivetech.com>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>