v1:
add a new enumeration type to identify device attribute node,
this method is relatively more efficient compared with 'strcmp' in
update_attr() function.

v2:
rename device_attr_type to device_attr_id.
Signed-off-by: Yang Wang <kevinyang.wang@amd.com>
Reviewed-by: Ma Jun <majun@amd.com>
Reviewed-by: Lijo Lazar <lijo.lazar@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>