msm: pm-8x60: Add retention mode for processor cores

Application processors can do retention mode (apps sleep) in which
the processor reduces the voltage and gates the clock and when an
interrupt occurs will bail out of the low power mode and resume from the
command after WFI.

In retention mode, processor core is not reset and so will not do a warm
boot from TZ.

Change-Id: Id035602dfc53edde88a17ea194dcc85df877e9ea
Signed-off-by: Praveen Chidambaram <pchidamb@codeaurora.org>