Check presence of fix for errata 843419 in Cortex-A53

A fix for errata 843419 may be available in revision r0p4 of the
Cortex-A53 processor. The presence of the fix is determined by checking
bit 8 in the REVIDR register.

If the fix is present we report ERRATA_NOT_APPLIES which silences the
erroneous 'missing workaround' warning.

Change-Id: Ibd2a478df3e2a6325442a6a48a0bb0259dcfc1d7
Signed-off-by: Jonathan Wright <jonathan.wright@arm.com>
diff --git a/lib/cpus/aarch64/cpu_helpers.S b/lib/cpus/aarch64/cpu_helpers.S
index 5a9226d..9f13ed2 100644
--- a/lib/cpus/aarch64/cpu_helpers.S
+++ b/lib/cpus/aarch64/cpu_helpers.S
@@ -198,6 +198,8 @@
  * Compare the CPU's revision-variant (x0) with a given value (x1), for errata
  * application purposes. If the revision-variant is less than or same as a given
  * value, indicates that errata applies; otherwise not.
+ *
+ * Shall clobber: x0-x3
  */
 	.globl	cpu_rev_var_ls
 func cpu_rev_var_ls
@@ -212,6 +214,8 @@
  * Compare the CPU's revision-variant (x0) with a given value (x1), for errata
  * application purposes. If the revision-variant is higher than or same as a
  * given value, indicates that errata applies; otherwise not.
+ *
+ * Shall clobber: x0-x3
  */
 	.globl	cpu_rev_var_hs
 func cpu_rev_var_hs