fix(morello): dts: fix GICv3 compatible string

The official GICv3 DT bindings require only a limited number of
compatible string, and disavowes the naming of an implementation.
Linux' "make dtbs_check" reports:
============
.../morello-soc.dt.yaml: interrupt-controller@2c010000: compatible: 'oneOf' conditional failed, one must be fixed:
        ['arm,gic-600', 'arm,gic-v3'] is too long
        'arm,gic-600' is not one of ['qcom,msm8996-gic-v3']
        'arm,gic-v3' was expected
        From schema: Documentation/devicetree/bindings/interrupt-controller/arm,gic-v3.yaml
============

Drop the redundant (because runtime detectable) and undocumented
implementation version, and just use the standard compatible string.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Change-Id: I05b207df271d6aa5bf3f2163f99ac0c594204c75