pwm: Update DT bindings to reference pwm.txt for cells documentation

The PWM client cells format is documented in the generic pwm.txt
documentation and duplicated in all PWM driver bindings. Remove
duplicate information and reference pwm.txt instead.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>