powerpc/e500mc: Add support for the wait instruction in e500_idle

e500mc cannot doze or nap due to an erratum (as well as having a
different mechanism than previous e500), but it has a "wait" instruction
that is similar to doze.

On 64-bit, due to the soft-irq-disable mechanism, the existing
book3e_idle should be used instead.

Signed-off-by: Vakul Garg <vakul@freescale.com>
Signed-off-by: Scott Wood <scottwood@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>