mailbox: k3-sec-proxy: Add DM to DMSC communication thread

R5 SPL would need to talk to DMSC using DM to DMSC sec-proxy threads.
Mark these as valid threads in the driver.

Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
Link: https://lore.kernel.org/r/20210607141753.28796-2-vigneshr@ti.com