interrupt_controller: gicv3: add Interrupt Translation Service support

This implements support for the optional Interrupt Translation Service
(ITS) module of the GICv3 Interrupt Controller.

The current implementation is designed for MSI/MSI-X interrupt delivery
in mind.
The gicv3 driver calls each ITS INVALL command when LPI interrupts are
enabled/disabled.
A simple atomic integer is used to allocate unique LPI INTIDs to ITS
users.
CPUs numbers are directly mapped as ICIDs into the Collections Table.

As a limitation it doesn't support indirect Device table to simplify
implementation but may use a large amount of memory.

INV, DISCARD, MOVI and MOVALL commands are not implemented.

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