feat(debugv8p9): add support for FEAT_Debugv8p9

This patch enables FEAT_Debugv8p9 and prevents EL1/0 from
trapping to EL3 when accessing MDSELR_EL1 register by
setting the MDCR_EL3.EBWE bit.

Signed-off-by: Arvind Ram Prakash <arvind.ramprakash@arm.com>
Change-Id: I3613af1dd8cb8c0d3c33dc959f170846c0b9695a
diff --git a/docs/getting_started/build-options.rst b/docs/getting_started/build-options.rst
index 52a9317..c9c92fd 100644
--- a/docs/getting_started/build-options.rst
+++ b/docs/getting_started/build-options.rst
@@ -320,6 +320,12 @@
    The flag can take values 0 to 2, to align with the ``ENABLE_FEAT``
    mechanism. Default value is ``0``.
 
+- ``ENABLE_FEAT_DEBUGV8P9``: Numeric value to enable ``FEAT_DEBUGV8P9``
+   extension which allows the ability to implement more than 16 breakpoints
+   and/or watchpoints. This feature is mandatory from v8.9 and is optional
+   from v8.8. This flag can take the values of 0 to 2, to align with the
+   ``ENABLE_FEAT`` mechanism. Default value is ``0``.
+
 -  ``ENABLE_FEAT_DIT``: Numeric value to enable ``FEAT_DIT`` (Data Independent
    Timing) extension. It allows setting the ``DIT`` bit of PSTATE in EL3.
    ``FEAT_DIT`` is a mandatory  architectural feature and is enabled from v8.4