[rtl] minor edits and updates

[stnolting]

This is just another rtl update PR.

• the XIP module will now raise a bus exception if the software tries to write to the XIP memory-mapped flash (indicated as "device error" by the NEORV32 runtime environment)
• use a behavioral description of the ALU's adder/subtractor (seems to result in better logic; maybe because the synthesizer has more flexibility)
• minor code edits and cleanups