arm64: dts: ti: k3-j7200-main: Add mailbox cluster nodes

The J7200 Main NavSS block contains a Mailbox IP instance with
multiple clusters, and follows the same integration style as on
J721E SoCs.

Add all the Mailbox clusters as their own nodes under the 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: Nishanth Menon <nm@ti.com>
Reviewed-by: Praneeth Bajjuri <praneeth@ti.com>
Link: https://lore.kernel.org/r/20201026232637.15681-3-s-anna@ti.com