dt-bindings: arm: Split i.MX8M Mini NITROGEN SoM based boards

The NITROGEN SoM based boards have compatible string in the form:
"vendor,custom-board", "boundary,som", "fsl,soc"
Split the binding entry so this triplet can be matched on.

Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Marek Vasut <marex@denx.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>