aarch64: Restrict range of PRFM opcodes

aarch64: Restrict range of PRFM opcodes

In the register-index forms of PRFM, the unallocated prefetch opcodes
24-31 have been reused for the encoding of the new RPRFM instruction.
The PRFM opcode space is now capped at 23 for these forms.  The other
forms of PRFM are unaffected.