ARM: dts: imx6qdl-udoo: add support for powering off

UDOO board has a possibility to disable most of imx6 SoC power supplies via
"EN_5V" signal, which is connected to "NANDF_D4" pad.

This reduces current consumption after shutdown from ~350 mA to ~40 mA and
prevents a SoC watchdog from starting it again on its own after the
watchdog timeout expires (previously it was not possible to shut down the
SoC permanently if its watchdog was ever enabled since it was still ticking
after the shutdown).

Unfortunately, this does not extinguish the "Power" LED (as it is powered
from an always-on voltage regulator).

To power the SoC up again press the power button (SW2, the one with a long
shaft).

Signed-off-by: Maciej S. Szmigiero <mail@maciej.szmigiero.name>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>