[reggen] Support a multireg count of 0

[andreaskurth]

reggen currently doesn't support 0 instances of a multireg. It fails with the following error:

File "util/reggen/multi_register.py", line 132, in __init__                                                                                                                                                                        
    raise ValueError("Multireg {} has a count of {}, "                                                            
ValueError: Multireg INP_PRD_CNT_CTRL has a count of 0, which isn't positive.

Instead of this error, I'd expect reggen to completely omit a multireg with count 0.

The problem can be avoided if the count of the multireg is a template parameter: in that case, a % if template_parameter > 0: can be placed around the multireg.

[rswarbrick]

On the plus side, this should be pretty trivial to fix. An easy fix:

• Define a specialized EmptyMultiReg exception
• Throw it instead of spitting out that error if the count is zero.
• Construct the object in a try / except in RegBlock._handle_multireg.

I'm hoping that it's just an hour's work.