drm/msm/gpu: Add devfreq tuning debugfs

Make the handful of tuning knobs available visible via debugfs.

v2: select DEVFREQ_GOV_SIMPLE_ONDEMAND because for some reason
    struct devfreq_simple_ondemand_data depends on this

Signed-off-by: Rob Clark <robdclark@chromium.org>
Patchwork: https://patchwork.freedesktop.org/patch/517784/
Link: https://lore.kernel.org/r/20230110231447.1939101-2-robdclark@gmail.com
Reviewed-by: Chia-I Wu <olvaffe@gmail.com>