interrupt_controller: Add GICv3 ITS API

Add driver API for the Interrupt Translation Service GICv3 module.

The Interrupt Translation Service (ITS) translates an input EventID
from a device, identified by its DeviceID, and determines the
corresponding INTID for this input and target Redistributor + target
PE for that INTID to be delivered as LPI interrupt.

The API permits :
- allocating a new unique LPI interrupt number
- initializing support for a new DeviceID and the required EventIDs
- mapping a DeviceID + EventID to a LPI interrupt number
- triggering an LPI with a DeviceID + EventID

This API will be used by the PCIe MSI/MSI-X logic in another patchset.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>