dts:arm64:mx8m: Set cma size of mx8m to 320 MiB

By default, the cma size is set to 920MiB, which cannot be initialized on some of our some of our machines because the memory is too small. Previously, the cma value was bypassed by setting a suitable value in the kernel command line. To make the devicetree config usable, the cma size is set to 320 MiB. The value seems to be suitable for EGL surfaces and video playback. By default libimxdmabuffer (required for vpu video playback) expects the cma heap node to be available at /dev/dma_heap/linux,cma-uncached. This node is only created when the devicetree cma declaration is used. However, it seems to be possible to create a soft-link if the node is named differently.

BCS 746-001058