Avoided top-posting and resending.

+  /* temporal locality */
+  return (INTVAL(operands[1])) ? \"prfm\\tPSTL1KEEP, [%0, #0]\" : 
+\"prfm\\tPLDL1KEEP, [%0, #0]\"; }"
+  [(set_attr "type" "prefetch")]
+)
+

With the locality value received in the instruction pattern, I think it would be safe to handle them in prefetch instruction.
This helps especially AArch64 has prefetch instructions that can handle this locality.

+(define_insn "prefetch"
+  [(prefetch (match_operand:DI 0 "address_operand" "r")
+            (match_operand:QI 1 "const_int_operand" "n")
+            (match_operand:QI 2 "const_int_operand" "n"))]
+  ""
+  "*
+{
+  int locality = INTVAL (operands[2]);
+
+  gcc_assert (IN_RANGE (locality, 0, 3));
+
+  if (locality == 0)
+     /* non temporal locality */
+     return (INTVAL(operands[1])) ? \"prfm\\tPSTL1STRM, [%0, #0]\" : \"prfm\\tPLDL1STRM, [%0, #0]\";
+
+  /* temporal locality */
+  return (INTVAL(operands[1])) ? \"prfm\\tPSTL%2KEEP, [%0, #0]\" : \"prfm\\tPLDL%2KEEP, [%0, #0]\";
+}"
+  [(set_attr "type" "prefetch")]
+)
+

I also have attached a patch that implements the following. 
*	Prefetch with immediate offset in the range 0 to 32760 (multiple of 8). Added a predicate for this.
*	Prefetch with immediate offset - in the range -256 to 255 (Gets generated only when we have a negative offset. Generates prfum instruction). Added a predicate for this.
*	Prefetch with register offset. (modified for printing the locality)

Regards
Ganesh