msm: kgsl: Add GPU speed config support to KGSL

gpu-speed-conig is device tree property. KGSL driver
can read this property to load dynamic GPU powerlevels
based on the speed configuation.

Change-Id: I3066bfe45db1117452fb2bf608615cac710e173f
Signed-off-by: Hareesh Gundu <hareeshg@codeaurora.org>