aarch64: Add the SME2 saturating conversion instructions

There are two instruction formats here:

- SQCVT, SQCVTU and UQCVT, which operate on lists of two or
  four registers.

- SQCVTN, SQCVTUN and UQCVTN, which operate on lists of
  four registers.

include/opcode/aarch64.h

@@ -717,6 +717,7 @@ enum aarch64_insn_class
   sme_size_12_hs,
   sme_size_22,
   sme_size_22_hsd,
+  sme_sz_23,
   sme_str,
   sme_start,
   sme_stop,