feat(manifest): parse relative addresses in memory regions

FF-A v.1. REL0 Table 5.2 specifies that memory regions can use base
address, or relative offset.

This patch updates the manifest parser to parse a new, optional
`relative-address` property in memory regions.

Change-Id: I0a2396b41de660f98e6a96c6da7097b2db843648
Signed-off-by: Karl Meakin <karl.meakin@arm.com>
diff --git a/inc/hf/manifest.h b/inc/hf/manifest.h
index 1a8cf3b..ea3bc23 100644
--- a/inc/hf/manifest.h
+++ b/inc/hf/manifest.h
@@ -76,6 +76,7 @@
 	MANIFEST_ERROR_RXTX_SIZE_MISMATCH,
 	MANIFEST_ERROR_MEM_REGION_EMPTY,
 	MANIFEST_ERROR_MEM_REGION_UNALIGNED,
+	MANIFEST_ERROR_BASE_ADDRESS_AND_RELATIVE_ADDRESS,
 	MANIFEST_ERROR_MEM_REGION_OVERLAP,
 	MANIFEST_ERROR_MEMORY_MISSING,
 	MANIFEST_ERROR_MEM_REGION_INVALID,