drm/amd/pm: add two new sysfs nodes for vangogh

This patch is to add two new sysfs nodes for vangogh:
pp_dpm_dclk and pp_dpm_vclk.
The two sysfs nodes are similar to pp_dpm_fclk/memclk/socclk.
pp_dpm_dclk represents the DPM frequency of dcn unit.
pp_dpm_vclk represents the DPM frequency of vcn unit.

Signed-off-by: Xiaojian Du <Xiaojian.Du@amd.com>
Acked-by: Evan Quan <evan.quan@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>