ARM: dts: am437x-sk-evm: disable DDR regulator in rtc-only/poweroff mode

Without this, the memory will remain active during poweroff consuming
extra power. Please note revision 2.1 PMIC seems to fail when DCDC3
disable is attempted, so this is not done on that PMIC revision. The
PMIC revision checks in the regulator patches make sure of this.

Signed-off-by: Tero Kristo <t-kristo@ti.com>
Signed-off-by: Dave Gerlach <d-gerlach@ti.com>
Signed-off-by: Keerthy <j-keerthy@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>