arm64: dts: imx8mm-nitrogen-r2: add ecspi2 support

Add the description for ecspi2 support.

Signed-off-by: Adrien Grassein <adrien.grassein@gmail.com>
Reviewed-by: Krzysztof Kozlowski <krzk@kernel.org>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Reviewed-by: Marco Felsch <m.felsch@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>