soc: ti: wkup_m3_ipc: Send NULL dummy message instead of pointer message

mbox_send_message() sends a u32 bit message, not a pointer to a message.
We only convert to a pointer type as a generic type. If we want to send
a dummy message of 0, then simply send 0 (NULL).

Signed-off-by: Andrew Davis <afd@ti.com>
Link: https://lore.kernel.org/r/20240325165507.30323-1-afd@ti.com
Signed-off-by: Nishanth Menon <nm@ti.com>