drm/amd/pm: change pp_dpm_sclk/mclk/fclk attribute is RO for aldebaran

the following clock is only support voltage DPM, change attribute to RO:
1. pp_dpm_sclk
2. pp_dpm_mclk
3. pp_dpm_fclk

Signed-off-by: Kevin Wang <kevin1.wang@amd.com>
Reviewed-by: Lijo Lazar <lijo.lazar@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>