[POWERPC] Add 6xx-style HID0_SLEEP support.

This adds a function to put a 6xx/7xx/7xxx/83xx family CPU into sleep
mode, and return after an interrupt has occurred.  It expects to be
called with interrupts disabled, and returns with interrupts disabled.
Interrupts are enabled while the processor is asleep, but the interrupt
that wakes the processor is not handled; it is still pending when this
function returns.

Signed-off-by: Scott Wood <scottwood@freescale.com>
Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Paul Mackerras <paulus@samba.org>