aarch64: Pass aarch64_indexed_za to parsers

aarch64: Pass aarch64_indexed_za to parsers

ZA indices have more parts than most operands, so passing these
parts around individually is more awkward than for other operand
types.  Things aren't too bad at the moment, but SME2 adds two
further pieces: an offset range and a vector group size.

This patch therefore replaces arguments for the individual pieces
with a single argument for the index as a whole.