Add support to turn off Beaglebone with poweroff(8) or shutdown(8) -p.

To cut off the power we need to start the shutdown sequence by writing
the OFF bit on PMIC.

Once the PMIC is programmed the SoC needs to toggle the PMIC_PWR_ENABLE
pin when it is ready for the PMIC to cut off the power. This is done by
triggering the ALARM2 interrupt on SoC RTC.

The RTC driver only works in power management mode which means it won't
provide any kind of time keeping functionality. It only implements a way
to trigger the ALARM2 interrupt when requested.

Differential Revision: https://reviews.freebsd.org/D1489
Reviewed by: rpaulo
MFC after: 2 weeks