aarch64: Consolidate ZA slice parsing

aarch64: Consolidate ZA slice parsing

Now that parse_typed_reg checks the range of tile register numbers
and libopcodes checks the range of vector select offsets, there's
very little difference between the parsing of ZA tile indices,
ZA array indices, and PSEL indices.  The main one is that ZA
array indices don't currently allow "za" to be qualified,
but we need to remove that restriction for SME2.

This patch therefore consolidates all three parsers into a single
routine, parameterised by the type of register that they expect.