arm64: dts: fvp: Fix GIC compatible names

The GIC DT binding only allows certain combinations of DT compatible
strings. The somewhat awkward "arm,cortex-a15-gic", "arm,cortex-a9-gic"
is not among those.

Drop that combination of different "cortex" based strings used for the
models, and replace it with the more useful combination including
"arm,gic-400".

Link: https://lore.kernel.org/r/20200513103016.130417-9-andre.przywara@arm.com


Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>