dt-bindings: arm: gic: Allow combining arm,gic-400 compatible strings

The arm,gic-400 compatible is probably the best matching string for the
GIC in most modern SoCs, but was only introduced later into the kernel.
For historic reasons and to keep compatibility, some SoC DTs were thus
using a combination of this name and one of the older strings, which
currently the binding denies.

Add a stanza to the DT binding to allow "arm,gic-400", followed by
either "arm,cortex-a15-gic" or "arm,cortex-a7-gic". This fixes binding
compliance for quite some SoC .dtsi files in the kernel tree.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Rob Herring <robh@kernel.org>