powerpc/85xx/86xx: Add suspend/resume support

This patch adds suspend/resume support for MPC8540 and MPC8641D-
compatible CPUs. To reach sleep state, we just write the SLP bit
into the PM control and status register.

So far we don't support Deep Sleep mode as found in newer MPC85xx
CPUs (i.e. MPC8536). It can be relatively easy implemented though,
and for it we reserve 'mem' suspend type.

Signed-off-by: Anton Vorontsov <avorontsov@ru.mvista.com>
Acked-by: Scott Wood <scottwood@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>