ARM: dts: am57xx-idk-common: Add overide powerhold property

commit 8804755bfb1f3cbc003e4ebe99eac491672f354c upstream.

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>
Cc: Ben Hutchings <ben.hutchings@codethink.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>