aarch64: Use generic_armv8_a_prefetch_tune in generic_armv8_a.h

generic_armv8_a.h defines generic_armv8_a_prefetch_tune but still uses
generic_prefetch_tune in generic_armv8_a_tunings.

This patch updates the pointer to generic_armv8_a_prefetch_tune.

This patch was bootstrapped and regtested on aarch64-linux-gnu, no regression.

Signed-off-by: Soumya AR <soumyaa@nvidia.com>

gcc/ChangeLog:

	* config/aarch64/tuning_models/generic_armv8_a.h: Updated prefetch
	struct pointer.

gcc/config/aarch64/tuning_models/generic_armv8_a.h

@@ -184,7 +184,7 @@ static const struct tune_params generic_armv8_a_tunings =
   (AARCH64_EXTRA_TUNE_BASE
    | AARCH64_EXTRA_TUNE_CSE_SVE_VL_CONSTANTS
    | AARCH64_EXTRA_TUNE_MATCHED_VECTOR_THROUGHPUT),	/* tune_flags.  */
-  &generic_prefetch_tune,
+  &generic_armv8_a_prefetch_tune,
   AARCH64_LDP_STP_POLICY_ALWAYS,   /* ldp_policy_model.  */
   AARCH64_LDP_STP_POLICY_ALWAYS    /* stp_policy_model.  */
 };