soc: ti: pm33xx: Add a print while entering RTC only mode with DDR in self-refresh

Currently there is no way to distinguish if the SoC entered DS0
mode or the RTC only mode. Hence add a print before entering
the RTC only mode.

Signed-off-by: Keerthy <j-keerthy@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>