drm/amdgpu: add the common code to support mclk OD

This implements mclk OverDrive(OD) through sysfs.
The new entry pp_mclk_od is read/write. The value of input/output
is an integer of the overclocking percentage.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Eric Huang <JinHuiEric.Huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>