powerpc/64: Make stf barrier PPC_BOOK3S_64 specific.

NXP Book3E platforms are not vulnerable to speculative store
bypass, so make the mitigations PPC_BOOK3S_64 specific.
Signed-off-by: Diana Craciun <diana.craciun@nxp.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/kernel/security.c

@@ -176,6 +176,7 @@ ssize_t cpu_show_spectre_v2(struct device *dev, struct device_attribute *attr, c
 	return s.len;
 }
 
+#ifdef CONFIG_PPC_BOOK3S_64
 /*
  * Store-forwarding barrier support.
  */
@@ -323,3 +324,4 @@ static __init int stf_barrier_debugfs_init(void)
 }
 device_initcall(stf_barrier_debugfs_init);
 #endif /* CONFIG_DEBUG_FS */
+#endif /* CONFIG_PPC_BOOK3S_64 */