arm64: dts: imx8mm: Remove incorrect fallback compatible for ocotp

Compared to i.MX7D, i.MX8MM has different ocotp layout, so it should
NOT use "fsl,imx7d-ocotp" as ocotp's fallback compatible, remove it.

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