dt-bindings: pwm: mxs: Document the clocks property

The 'clocks' property is mandatory for the PWM to operate.

Document it.

Signed-off-by: Fabio Estevam <festevam@denx.de>
Reviewed-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>