ARM: dts: am43xx-epos-evm: Keep DCDC3 regulator on in suspend to memory

When going to suspend to ram mode (or rtc-only mode), the DDR regulator
must be told to stay on, else this rail will go down when the PMIC_EN
signal is deasserted.

Signed-off-by: Mike Erdahl <m-erdahl@ti.com>
Signed-off-by: Andrew F. Davis <afd@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>