ARM: dts: imx: add nvmem property for cpu0

Add nvmem related property for cpu0, then nvmem API could be used
to read cpu speed grading to avoid directly read OCOTP registers
mapped which could not handle defer probe.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>