Input: pwm-beeper - add devicetree probing support

A very simple binding, the only property is the phandle to the PWM.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Reviewed-by: Thierry Reding <thierry.reding@avionic-design.de>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>