Cell #3 (usr_id) - mailbox user id for identifying the interrupt line
associated with generating a tx/rx fifo interrupt.
+Optional Properties:
+--------------------
+- ti,mbox-send-noirq: Quirk flag to allow the client user of this sub-mailbox
+ to send messages without triggering a Tx ready interrupt,
+ and to control the Tx ticker. Should be used only on
+ sub-mailboxes used to communicate with WkupM3 remote
+ processor on AM33xx/AM43xx SoCs.
+
Mailbox Users:
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A device needing to communicate with a target processor device should specify