dt-bindings: arm: imx: add imx8mm nitrogen support

The Nitrogen8M Mini is an ARM based single board computer (SBC).

Signed-off-by: Adrien Grassein <adrien.grassein@gmail.com>
Reviewed-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>