tests: arm64: add GICv3 ITS test suite

Add a simple ITS driver test allocating 8192 LPIs over 256 DeviceIDS
and 32 EventIDs per DeviceID to exercise the ITS for the MSI/MSI-X
usecase.

The test is set as `build_only: true` and whitelisted to the
fvp_base_revc_2xaemv8a board until the shipped Zephyr QEMU
has the ITS support.
Since `fvp_base_revc_2xaemv8a` needs FIP & BL1 files, test is marked
as `skip: true`.

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