board: fix compatible property Milk-V Mars CM

For the Milk-V Mars CM (lite) we have only been copying sizeof(void *)
bytes to the compatible property instead of the whole string list.

Fixes: de322959 ("board: add support for Milk-V Mars CM")
Reported-by: E Shattow <lucent@gmail.com>
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>