dt-bindings: input: pwm-beeper: convert to dt schema

Convert the binding doc to dt schema, and also fixed the
example from fixed-regulator to regulator-fixed.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20230407075259.1593739-1-peng.fan@oss.nxp.com


Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>