ARM: dts: imx6dl-colibri: Command pmic to standby for poweroff

The Colibri iMX6 HW doesn't allow to use the PWR_ON_REQ signal for
poweroff. Use the fsl,pmic-stby-poweroff property to command the PMIC
into a low power mode in poweroff.

Signed-off-by: Max Krummenacher <max.krummenacher@toradex.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>