arm64: dts: imx8m: Document the fuse address calculation

The mapping from OCOTP reg DT property to Fusemap Descriptions Table in
the datasheet is often unclear. Add a comment to make it easier to find
out how it works. No functional change.

Reviewed-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Marek Vasut <marex@denx.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>