pm: notify readclock of reboot(RBT_POWEROFF)
pm: notify readclock of reboot(RBT_POWEROFF)

Some ARM chips handle power-off with RTC alarms. PM notifies
readclock (the driver for RTCs) about the impending power-off.
If the power-off mechanism is an RTC alarm, readclock will
set the alarm. If not, there is no effect.

Change-Id: Iee00066def2a0f742cdf0dbde8e32b376edf1b78

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