dt-bindings: input: qpnp-power-on: add description for qcom,modem-reset

Add a description for the new property qcom,modem-reset.  It is
used to specify that a PON device can power off an attached modem
chip via its primary PMIC.

Change-Id: Ifce1a33f9a03968d2c8648f52db2bf5599814b31
Signed-off-by: David Collins <collinsd@codeaurora.org>