Input: pwm-beeper - support customized freq for SND_BELL

Extend the pwm-beeper driver to support customized frequency for SND_BELL
from device properties.

Signed-off-by: Guan Ben <ben.guan@cn.bosch.com>
Signed-off-by: Mark Jonas <mark.jonas@de.bosch.com>
Signed-off-by: Heiko Schocher <hs@denx.de>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>