dt-bindings: gpu: mali-midgard: Tidy up conversion to YAML

dt-bindings: gpu: mali-midgard: Tidy up conversion to YAML

Instead of grouping alphabetically by third-party vendor, leading to
one-element enums, start sorting by Mali model number, as done for
Utgard.

This already allows us to de-duplicate two "arm,mali-t760" sections and
will make it easier to add new vendor compatibles.

Fixes: 553cedf60056 ("dt-bindings: Convert Arm Mali Midgard GPU to DT schema")
Fixes: 1be5b54d26ae ("dt-bindings: gpu: mali-midgard: Add samsung exynos5250 compatible")
Cc: Rob Herring <robh@kernel.org>
Signed-off-by: Andreas Färber <afaerber@suse.de>
[robh: don't resort everything to avoid conflicts]
Signed-off-by: Rob Herring <robh@kernel.org>