firmware: ti_sci: Add support for IRQ management

TISCI abstracts the handling of IRQ routes where interrupt sources
are not directly connected to host interrupt controller. Add support
for the set of TISCI commands for requesting and releasing IRQs.

Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
Acked-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>