drm/msm: add max-freq gpu param to uapi

We need this in userspace for interpreting some of the perf ctrs.

Note possibly not quite sufficient if we had some frequency mgmt
approach other than race-to-idle.  Not really sure what the best
thing to do if we did.  Although displaying results as a percentage
of max frequence seems sensible(ish) if we did.

Signed-off-by: Rob Clark <robdclark@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>