arm64: dts: imx: Add i.mx8mm nitrogen basic dts support

Tested with a basic Build Root configuration booting from sdcard.

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