dtoc: Process driver aliases along with drivers

dtoc: Process driver aliases along with drivers

Instead of using a separate step for this processing, handle it while
scanning its associated driver. This allows us to drop the code coverage
exception in this case.

Note that only files containing drivers are scanned by dtoc, so aliases
declared in a file that doesn't hold a driver will not be noticed. It
would be confusing to put them anywhere other than in the driver that they
relate to, but update the documentation to say this explicitly, just in
case.

Signed-off-by: Simon Glass <sjg@chromium.org>