Check presence of fix for errata 835769 in Cortex-A53

A fix for errata 835769 may be available in revisions r0p2, r0p3 or r0p4
of the Cortex-A53 processor. The presence of the fix is determined by
checking bit 7 in the REVIDR register.

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

Change-Id: Ib75b008e755e9ac648554ca9398024fdbea4a91a
Signed-off-by: Jonathan Wright <jonathan.wright@arm.com>

lib/cpus/aarch64/cortex_a53.S

@@ -145,8 +145,23 @@ endfunc check_errata_855873
  * This workaround is statically enabled at build time.
  */
 func check_errata_835769
-	mov	x1, #0x04
-	b	cpu_rev_var_ls
+	cmp	x0, #0x04
+	b.hi	errata_not_applies
+	/*
+	 * Fix potentially available for revisions r0p2, r0p3 and r0p4.
+	 * If r0p2, r0p3 or r0p4; check for fix in REVIDR, else exit.
+	 */
+	cmp	x0, #0x01
+	mov	x0, #ERRATA_APPLIES
+	b.ls	exit_check_errata_835769
+	/* Load REVIDR. */
+	mrs	x1, revidr_el1
+	/* If REVIDR[7] is set (fix exists) set ERRATA_NOT_APPLIES, else exit. */
+	tbz	x1, #7, exit_check_errata_835769
+errata_not_applies:
+	mov	x0, #ERRATA_NOT_APPLIES
+exit_check_errata_835769:
+	ret
 endfunc check_errata_835769
 
 /*