arm64: dts: ti: k3-j721e-main: Add mailbox cluster nodes

The J721E Main NavSS block contains a Mailbox IP instance with
multiple clusters. Each cluster is equivalent to an Mailbox IP
instance on OMAP platforms.

Add all the Mailbox clusters as their own nodes under the MAIN
NavSS cbass_main_navss interconnect node instead of creating an
almost empty parent node for the new K3 mailbox IP and the clusters
as its child nodes. All these nodes are enabled by default in the
base dtsi file, but any cluster that does not define any child
sub-mailbox nodes should be disabled in the corresponding board
dts files.

NOTE:
The NavSS only has a limited number of interrupts, so none of the
interrupts generated by a Mailbox IP are added by default. Only
the needed interrupts that are targeted towards the A72 GIC will
have to be added later on in the board dts files alongside the
corresponding sub-mailbox child nodes.

Signed-off-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Tero Kristo <t-kristo@ti.com>