arm64: dts: imx8mp: add fsl,nominal-mode property into nominal.dtsi

arm64: dts: imx8mp: add fsl,nominal-mode property into nominal.dtsi

The imx8mp-nominal.dtsi is meant to be included into boards that want to
override the default overdrive clock settings with settings suitable for
running in nominal drive mode at its lower required voltage.

Specifying fsl,operating-mode = "nominal" informs drivers of this fact,
so they can sanity check runtime clock reconfiguration to observe the
limits imposed by nominal mode.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Reviewed-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>