dt-bindings: soc: imx8m: add DT Binding doc for soc unique ID

Add DT Binding doc for the Unique ID of i.MX 8M series.

Reviewed-by: Krzysztof Kozlowski <krzk@kernel.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Alice Guo <alice.guo@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>