drm/panfrost: Register devfreq cooling device

When we have devfreq, also try to register a basic cooling device in
case GPU workloads manage to hit thermal throttling thresholds.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/21f228099321f460d62e0ab7c77b2d2213dd4da8.1574974319.git.robin.murphy@arm.com