arm64: dts: imx8mn: Use "fsl,imx8mm-ocotp" as ocotp's fallback compatible

Use "fsl,imx8mm-ocotp" as i.MX8MN ocotp's fallback compatible instead
of "fsl,imx7d-ocotp" to support SoC UID read, as i.MX8MN reuses
i.MX8MM's SoC ID driver.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>