Implement clock and power gating support for tonga.  On Tonga
this is handles by the SMU rather than direct register settings
in the driver.
Signed-off-by: Rex Zhu <Rex.Zhu@amd.com>
Reviewed-by: Jammy Zhou <Jammy.Zhou@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>