pwm: Add can_sleep property to drivers

Calls to PWM drivers connected through I2C can sleep.
Use the new can_sleep property.

Acked-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Florian Vaussard <florian.vaussard@epfl.ch>
Signed-off-by: Thierry Reding <thierry.reding@avionic-design.de>