dtoc: Set up the uclasses that are used

dtoc: Set up the uclasses that are used

We only care about uclasses that are actually used. This is determined by
the drivers that use them. Check all the used drivers and build a list of
'valid' uclasses.

Also add references to the uclasses so we can generate C code that uses
them. Attach a uclass to each valid driver.

For the tests, now that we have uclasses we must create an explicit test
for the case where a node does not have one. This should only happen if
the source code does not build, or the source-code scanning fails to find
it.

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