broadcom/genxml: Support address fields with <32 bits

broadcom/genxml: Support address fields with <32 bits

I was writing the XML such that the address field overlapped various flags
in the alignment bits, which caused pain when trying to unpack for decode.
Instead, keep the XML matching the docs (address fields don't overlap),
and just infer the appropriate shift value during decode.

During pack, the address is just applied to the appropriate bits
already, ignoring the sub-byte start/end fields.