Input: pmic8xxx-pwrkey - support shutdown

On pm8xxx PMICs, shutdown and restart are signaled to the PMIC via a pin
called PS_HOLD. When this pin goes low, the PMIC performs a configurable
power sequence. Add a .shutdown hook so that we can properly configure this
power sequence for shutdown or restart depending on the system state.

Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
Reviewed-by: Bjorn Andersson <bjorn.andersson@sonymobile.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>