broadcom/vc5: Add some comments about the texture/output format ordering.

broadcom/vc5: Add some comments about the texture/output format ordering.

The output formats are consistent with their channels appearing from low
to high in their name.  Textures are interpreted the same way, but their
names may have the channels swapped around.  I'm retaining the texture
names so that we are consistent with the documentation, but I want to
leave a warning for others.