SMCCC/PCI: Add initial PCI conduit definitions

Add constants, structures and build definition for the
new standard SMCCC PCI conduit. These are documented
in DEN0115A.

https://developer.arm.com/documentation/den0115/latest

Signed-off-by: Jeremy Linton <jeremy.linton@arm.com>
Change-Id: If667800a26b9ae88626e8d895674c9c2e8c09658
diff --git a/make_helpers/defaults.mk b/make_helpers/defaults.mk
index 8d0cd04..0433123 100644
--- a/make_helpers/defaults.mk
+++ b/make_helpers/defaults.mk
@@ -212,6 +212,9 @@
 # True Random Number firmware Interface
 TRNG_SUPPORT            	:= 0
 
+# SMCCC PCI support
+SMC_PCI_SUPPORT            	:= 0
+
 # Whether code and read-only data should be put on separate memory pages. The
 # platform Makefile is free to override this value.
 SEPARATE_CODE_AND_RODATA	:= 0