add sysfs attr to read/write smartshift bias level.
document smartshift_bias sysfs attr.

V2: add attr to amdgpu_device_attrs and use attr_update (Lijo)
Signed-off-by: NSathishkumar S <sathishkumar.sundararaju@amd.com>
Reviewed-by: NLijo Lazar <lijo.lazar@amd.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>