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.
