drm/amdgpu: support rom clockgating related function for NV family

Add functions to support enable/disable rom clock gating and get rom
clock gating status.

Signed-off-by: Likun Gao <Likun.Gao@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>