binman: Correct use of 'replace' in IFWI tests

At present the Intel IFWI entry uses 'replace' without the 'ifwi-' prefix.
This is a fairly generic name which might conflict with the main Entry
base class at some point, if more features are added. Add a prefix.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/binman/README.entries b/tools/binman/README.entries
index 0f0e367..11c55fd 100644
--- a/tools/binman/README.entries
+++ b/tools/binman/README.entries
@@ -432,6 +432,12 @@
 Each subnode describes an entry which is placed into the IFWFI with a given
 sub-partition (and optional entry name).
 
+Properties for subnodes:
+    ifwi-subpart - sub-parition to put this entry into, e.g. "IBBP"
+    ifwi-entry - entry name t use, e.g. "IBBL"
+    ifwi-replace - if present, indicates that the item should be replaced
+        in the IFWI. Otherwise it is added.
+
 See README.x86 for information about x86 binary blobs.