fix(cpus): workaround for Cortex-X3 erratum 2070301

Cortex-X3 erratum 2070301 is a Cat B erratum that applies to all
revisions <= r1p2 and is still open.
The workaround is to write the value 4'b1001 to the PF_MODE bits
in the IMP_CPUECTLR2_EL1 register. This places the data prefetcher
in the most conservative mode instead of disabling it.

SDEN documentation:
https://developer.arm.com/documentation/2055130/latest

Change-Id: I337c4c7bb9221715aaf973a55d0154e1c7555768
Signed-off-by: Sona Mathew <sonarebecca.mathew@arm.com>
diff --git a/docs/design/cpu-specific-build-macros.rst b/docs/design/cpu-specific-build-macros.rst
index bf04558..ad05a50 100644
--- a/docs/design/cpu-specific-build-macros.rst
+++ b/docs/design/cpu-specific-build-macros.rst
@@ -726,6 +726,10 @@
 
 For Cortex-X3, the following errata build flags are defined :
 
+- ``ERRATA_X3_2070301``: This applies errata 2070301 workaround to the Cortex-X3
+  CPU. This needs to be enabled only for revisions r0p0, r1p0, r1p1 and r1p2 of
+  the CPU and is still open.
+
 - ``ERRATA_X3_2313909``: This applies errata 2313909 workaround to
   Cortex-X3 CPU. This needs to be enabled only for revisions r0p0 and r1p0
   of the CPU, it is fixed in r1p1.