ARM: dts: imx6ul-liteboard: Support poweroff

Support proper system power-off, which disables main regulator. This
results in much lower power consumption and support of power-on issued
by button press.

Signed-off-by: Marcin Niestroj <m.niestroj@grinn-global.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>