ARM: dts: imx6ul: add pm_power_off support

i.MX6UL can be powered off by programming SNVS.

When long press ON/OFF button(5 seconds),
PMIC_ON_REQ pin will be set to low and external
PMIC will be powered off.

And system can be powered on by long press ON/OFF
button again.

Signed-off-by: Anson Huang <b20788@freescale.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>