readclock: work-in-progress

Need working RTC for PMIC power-off. This commit is a first pass
at implementing support for the RTC in the AM335X SoC in readclock.
To support the xM, a driver for the xM's PMIC needs to be written.

TODO: Needs extensive testing across all platforms.
tps65217: work in progress

Remaining Tasks:

 - Probably need to factor out do_shutdown into libsys.
kernel: work-in-progress

Remaining Tasks:

 - Need to freeze RTC in readclock and then re-enable it in the kernel
when we're ready to power off. I've been getting segfaults and need
to figure out how to unprotect the page for the RTC.
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: I873cfcf0661a8a9eb50c9a89a3f55d791c43475a