ARM: dts: am57xx-beagle-x15-common: Add overide powerhold property

The PMICs have POWERHOLD set by default which prevents PMIC shutdown
even on DEV_CTRL On bit set to 0 as the Powerhold has higher priority.
So to enable pmic power off this property lets one over ride the default
value and enable pmic power off.

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