ARM: dts: imx6qdl: Enable fsl,sec-v4.0-pwrkey

The imx6q Technical reference manual shows the interrupt is
available to wake from sleep using the power button. The driver
has been available for quite some time, and other variants of the
i.MX6 have it enabled, so this implements it much like the others.

Signed-off-by: Adam Ford <aford173@gmail.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>