Add support for pointer authentication

The previous commit added the infrastructure to load and save
ARMv8.3-PAuth registers during Non-secure <-> Secure world switches, but
didn't actually enable pointer authentication in the firmware.

This patch adds the functionality needed for platforms to provide
authentication keys for the firmware, and a new option (ENABLE_PAUTH) to
enable pointer authentication in the firmware itself. This option is
disabled by default, and it requires CTX_INCLUDE_PAUTH_REGS to be
enabled.

Change-Id: I35127ec271e1198d43209044de39fa712ef202a5
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>
diff --git a/make_helpers/defaults.mk b/make_helpers/defaults.mk
index 6422397..819abcd 100644
--- a/make_helpers/defaults.mk
+++ b/make_helpers/defaults.mk
@@ -87,6 +87,9 @@
 # Flag to enable exception handling in EL3
 EL3_EXCEPTION_HANDLING		:= 0
 
+# Flag to enable Pointer Authentication
+ENABLE_PAUTH			:= 0
+
 # Build flag to treat usage of deprecated platform and framework APIs as error.
 ERROR_DEPRECATED		:= 0