ARM: dts: j72xx-r5-common-proc-board: Add DM firmware node

Add DM firmware node which will provide DM services during R5 SPL stage.

Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
Link: https://lore.kernel.org/r/20210607141753.28796-5-vigneshr@ti.com