readclock: work-in-progress

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

XXX: segfault in the kernel. need to find a solution...

tps65217(info)Power Button Pressed
MINIX will now be shut down ...
MINIX has halted and will now power off.
PMIC Power-Off in 5 Seconds
pagefault in kernel at pc 0xf0423d9c address 0x44e3e040
cpu 0 is_nested = 1 scheduled was: process -4 (idle0), pc = 0x0
idle0        -4 0x0
kernel panic: Unhandled kernel exception
kernel on CPU 0: 0x0
MINIX will now reset.
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