This is the final step to convert serial to the new driver model.
The dev->driver_api is now being used by individual drivers.
As a side effect, it is now possible to build multiple serial
drivers in the final image (though it won't make much sense).
Change-Id: I5d864e6503a5431b29006c311320155adf81cf5b
Signed-off-by: Daniel Leung <daniel.leung@intel.com>
Daniel Leung