docs(errata_abi): document the errata abi changes

Updated errata ABI feature enable flag and the errata non-arm
interconnect based flag, the default values for when the
feature is not enabled.

Change-Id: Ieb2144a1bc38f4ed684fda8280842a18964ba148
Signed-off-by: Sona Mathew <SonaRebecca.Mathew@arm.com>
diff --git a/docs/global_substitutions.txt b/docs/global_substitutions.txt
index 9428fe9..80012e7 100644
--- a/docs/global_substitutions.txt
+++ b/docs/global_substitutions.txt
@@ -69,3 +69,4 @@
 .. |UEFI| replace:: :term:`UEFI`
 .. |WDOG| replace:: :term:`WDOG`
 .. |XLAT| replace:: :term:`XLAT`
+.. |ERRATA_ABI| replace:: :term:`ERRATA_ABI`