ARM: dts: imx6ul-kontron-n6310-s: Disable the snvs-poweroff driver

ARM: dts: imx6ul-kontron-n6310-s: Disable the snvs-poweroff driver

The snvs-poweroff driver can power off the system by pulling the
PMIC_ON_REQ signal low, to let the PMIC disable the power.
The Kontron SoMs do not have this signal connected, so let's remove
the node.

This fixes a real issue when the signal is asserted at poweroff,
but not actually causing the power to turn off. It was observed,
that in this case the system would not shut down properly.

Signed-off-by: Frieder Schrempf <frieder.schrempf@kontron.de>
Fixes: 1ea4b76cdfde ("ARM: dts: imx6ul-kontron-n6310: Add Kontron i.MX6UL N6310 SoM and boards")
Signed-off-by: Shawn Guo <shawnguo@kernel.org>