drm/amdgpu: Add CLK IP base offset

so we can read/write the registers in CLK domain
through RREG32/WREG32_SOC15

Reviewed-by: Evan Quan <evan.quan@amd.com>
Signed-off-by: Rex Zhu <Rex.Zhu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>