soc: qcom: aop_msg: Add support for sending DDR freq msg to AOP

To ensure that a clean power off is performed, APPS must
communicate to AOP to lower the DDR frequency. Add
functionality for using the mailbox interface to communicate
the desired DDR frequency with AOP when powering off the
device.

Change-Id: I404ca770e1a3413e0febfd1f446fe01113600dc0
Signed-off-by: Isaac J. Manjarres <isaacm@codeaurora.org>