arm64: dts: ti: k3-am64-main: Add mailbox cluster nodes

The AM64 MAIN domain contains a Mailbox IP instance with multiple
clusters, and is a variant of the IP on current AM65x and J721E
SoCs. The AM64x SoC has only 8 clusters with no interrupts routed
to the A53 core on the first 2 clusters. The interrupt outputs
from the IP do not go through any Interrupt Routers and are
hard-wired to each processor, with only couple of interrupts
from each cluster reaching the A53 core.

Add all the Mailbox clusters that generate interrupts towards the
A53 core as their own nodes under the cbass_main node instead of
creating an almost empty parent node for the 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.

Signed-off-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Nishanth Menon <nm@ti.com>
Reviewed-by: Gowtham Tammana <g-tammana@ti.com>
Link: https://lore.kernel.org/r/20210322185430.957-3-s-anna@ti.com