Add support for accessing DB/DI registers via virtual __DBR operator

Signed-off-by: Michael Buesch <m@bues.ch>
Fix AUF/OPN for instance DBs

Signed-off-by: Michael Buesch <m@bues.ch>
In __DBR operator return 0 for uninitialized DBR

Signed-off-by: Michael Buesch <m@bues.ch>
Add test for DB/DI register assignments

Signed-off-by: Michael Buesch <m@bues.ch>