readclock: add support for am335x RTC

Add support for getting/setting the am335x SoC's internal real
time clock. Also, allow the power off alarm to be set.

Make readclock an "always on" driver. This is needed for setting
power-off alarms whenever the power button is pressed on the BBB.

Replace the readclock.sh script & single run driver with a
readclock program that takes the same arguments and forwards
the requests on to the always up readclock driver.
tps65217: work in progress

Remaining Tasks:

 - Probably need to factor out do_shutdown into libsys.
kernel: add support for am335x PMIC-base power-off

AM335X SoCs support power-off via a power management
chip (PMIC). An alarm in the real time clock is used
to trigger the PMIC to cut the power.

To ensure the alarm goes off when the system is in
a safe state, the RTC is frozen when the alarm is
set. At the last moment, the kernel unfreezes the RTC
to let the alarm go off.

This patch adds a mini driver for unfreezing the RTC
as well as code to handle RBT_POWEROFF on Minix/arm.
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