dt-bindings: input: qpnp-power-on: Add qcom,log-kpd-event description

Add support 'qcom,log-kpd-event' property which when enabled causes
KPDPWR status to be logged during driver INIT and runtime when key
is pressed.

Change-Id: Ib7b78bc602d84262e107de3d346a9e3307e2a9be