irqchip/ti-sci-inta: Add support for unmapped event handling

The DMA (BCDMA/PKTDMA and their rings/flows) events are under the INTA's
supervision as unmapped events in AM64.

In order to keep the current SW stack working, the INTA driver must replace
the dev_id with it's own when a request comes for BCDMA or PKTDMA
resources.

Implement parsing of the optional "ti,unmapped-event-sources" phandle array
to get the sci-dev-ids of the devices where the unmapped events originate.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20201020073243.19255-3-peter.ujfalusi@ti.com