arm64: dts: imx8mm-nitrogen-r2: add wifi/bt chip

Add usdhc3 description which corresponds to the wifi/bt chip

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