dt-bindings: display: msm: Convert GMU bindings to YAML

Convert display/msm/gmu.txt to display/msm/gmu.yaml and remove the old
text bindings.  The 'sram' text from the old binding never applied to
the GMU so it was not converted but all the other properties were correct.

Acked-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@chromium.org>