Add support for unique_id and serial_number, as these are now
the same value, and will be for future ASICs as well.

v2: Explicitly create unique_id only for VG10/20/ARC
v3: Change set_unique_id to get_unique_id for clarity
Signed-off-by: Kent Russell <kent.russell@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>