dt-bindings: input: pwm-vibrator: Add enable-gpio

Some pwm vibrators have a dedicated enable GPIO that needs to be set
high so that the vibrator works. Document that.

Signed-off-by: Luca Weiss <luca@z3ntu.xyz>
Reviewed-by: Sebastian Reichel <sre@kernel.org>
Reviewed-by: Brian Masney <bmasney@redhat.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Caleb Connolly <caleb.connolly@linaro.org>
Link: https://lore.kernel.org/r/20230427-hammerhead-vibra-v1-1-e87eeb94da51@z3ntu.xyz
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>