msm: pm: Add support for retention on each core

Allow each core on 8974 to enter retention and retain
the state of the CPU. On 8960 as hardware restriction does
not permit all cores to enter retention at the same time,
allow only core 0 to enter retention when all other cores
are offline to maintain backward compatibility.

Change-Id: I6690e901afc9fa3a27010c264fa3acc9a0722c1a
Signed-off-by: Priyanka Mathur <pmathur@codeaurora.org>