arm64: dts: imx8mm-kontron: Add spidev support

Allow the usage of ecspi3 in userspace. This is required for
compatibility with the v5.4-ktn tree.

Signed-off-by: Fabio Estevam <festevam@denx.de>
[adjust commit message]
Signed-off-by: Frieder Schrempf <frieder.schrempf@kontron.de>