arm64: dts: ti: k3*: Introduce reg definition for interrupt routers

Interrupt routers are memory mapped peripherals, that are organized
in our dts bus hierarchy to closely represents the actual hardware
behavior.

However, without explicitly calling out the reg property, using
2021.03+ dt-schema package, this exposes the following problem with
dtbs_check:

/arch/arm64/boot/dts/ti/k3-am654-base-board.dt.yaml: bus@100000:
interrupt-controller0: {'type': 'object'} is not allowed for
{'compatible': ['ti,sci-intr'], .....

Even though we don't use interrupt router directly via memory mapped
registers and have to use it via the system controller, the hardware
block is memory mapped, so describe the base address in device tree.

This is a valid, comprehensive description of hardware and permitted
by the existing ti,sci-intr schema.

Reviewed-by: Tero Kristo <kristo@kernel.org>
Reviewed-by: Lokesh Vutla <lokeshvutla@ti.com>
Signed-off-by: Nishanth Menon <nm@ti.com>
Link: https://lore.kernel.org/r/20210511194821.13919-1-nm@ti.com