arm64: dts: imx8mm/n: Remove the 'pm-ignore-notify' property

The 'pm-ignore-notify' property is not a valid property and there is
no documentation for it.

Drop such invalid property.

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