board: add support for Milk-V Mars CM

We already support the VisionFive 2 and the Milk-V Mars board by
patching the VisionFive 2 device tree. With this patch the same
is done for the Milk-V Mars CM.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Tested-by: E. Shattow <lucent@gmail.com>
Reviewed-by: E. Shattow <lucent@gmail.com>