dt-bindings: mailbox: omap: Update binding for AM64x SoCs

Update the existing OMAP Mailbox binding to include the info for
AM64x SoCs. There are some minor IP integration differences between
the AM64x SoCs and the previous AM65x and J721E SoC families.
Signed-off-by: Suman Anna <s-anna@ti.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>

Documentation/devicetree/bindings/mailbox/omap-mailbox.txt

@@ -28,6 +28,9 @@ SoCs has each of these instances form a cluster and combine multiple clusters
 into a single IP block present within the Main NavSS. The interrupt lines from
 all these clusters are multiplexed and routed to different processor subsystems
 over a limited number of common interrupt output lines of an Interrupt Router.
+The AM64x SoCS also uses a single IP block comprising of multiple clusters,
+but the number of clusters are smaller, and the interrupt output lines are
+connected directly to various processors.
 
 Mailbox Device Node:
 ====================
@@ -42,6 +45,7 @@ Required properties:
 			    "ti,omap4-mailbox" for OMAP44xx, OMAP54xx, AM33xx,
 						   AM43xx and DRA7xx SoCs
 			    "ti,am654-mailbox" for K3 AM65x and J721E SoCs
+			    "ti,am64-mailbox" for K3 AM64x SoCs
 - reg:			Contains the mailbox register address range (base
 			address and length)
 - interrupts:		Contains the interrupt information for the mailbox