v3d: Add pack/unpack/decode support for fields with a "- 1" modifier.

v3d: Add pack/unpack/decode support for fields with a "- 1" modifier.

Right now, we name these fields as "field name minus one" so that your C
code obviously states what the value should be.  However, it's easy enough
to handle at the codegen level with another little XML attribute, meaning
less C code and easier-to-read values in CLIF dumping and gdb as well.

(The actual CLIF format for simulator and FPGA replay takes in
pre-minus-one values, so we need it there too).