ARM: dts: imx6: add pm_power_off support for i.mx6 chips

All chips of i.mx6 can be powered off by programming SNVS.
For example :
On i.mx6q-sabresd board, PMIC_ON_REQ connect with external
pmic ON/OFF pin, that will cause the whole PMIC powered off
except VSNVS. And system can restart once PMIC_ON_REQ goes
high by push POWRER key.

Signed-off-by: Robin Gong <b38343@freescale.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>