refactor(fvp): use more arm generic code for gicv3

The arm generic implementation for the GIC is quite comprehensive and
the fvp's requirements don't diverge too much. Despite that, they
completely override a lot of code that is effectively reused. Use the
generic implementation instead to make it easier to follow and override
as little code as possible.

Change-Id: I3474aa970d7fbb91d75c0be6a255bc0da734f860
Signed-off-by: Boyan Karatotev <boyan.karatotev@arm.com>
6 files changed