soc: ti: wkup_m3_ipc: Add rtc_only with ddr in self refresh mode support

Adds rtc_only support. This needs resume function to shutdown and
reboot the m3.

Signed-off-by: Keerthy <j-keerthy@ti.com>
Signed-off-by: Santosh Shilimkar <santosh.shilimkar@oracle.com>