With the system coprocessor managing the range allocation of the
inputs to Interrupt Aggregator, it is difficult to represent
the device IRQs from DT.

The suggestion is to use MSI in such cases where devices wants
to allocate and group interrupts dynamically.

Create a MSI domain bus layer that allocates and frees MSIs for
a device.

APIs that are implemented:
- ti_sci_inta_msi_create_irq_domain() that creates a MSI domain
- ti_sci_inta_msi_domain_alloc_irqs() that creates MSIs for the
  specified device and resource.
- ti_sci_inta_msi_domain_free_irqs() frees the irqs attached to the device.
- ti_sci_inta_msi_get_virq() for getting the virq attached to a specific event.
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>