drm/amdgpu: attr to control SS2.0 bias level (v2)

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: Sathishkumar S <sathishkumar.sundararaju@amd.com>
Reviewed-by: Lijo Lazar <lijo.lazar@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>