binman: Avoid setting the image_pos attribute directly

Two places set this attribute directly. Update them to use the function
provided.

Signed-off-by: Simon Glass <sjg@chromium.org>

tools/binman/etype/atf_fip.py

@@ -248,7 +248,7 @@ class Entry_atf_fip(Entry_section):
             fent = entry._fip_entry
             entry.size = fent.size
             entry.offset = fent.offset
-            entry.image_pos = self.image_pos + entry.offset
+            entry.SetImagePos(image_pos + self.offset)
 
     def ReadChildData(self, child, decomp=True, alt_format=None):
         if not self.reader:

tools/binman/etype/cbfs.py

@@ -245,7 +245,7 @@ class Entry_cbfs(Entry):
             cfile = entry._cbfs_file
             entry.size = cfile.data_len
             entry.offset = cfile.calced_cbfs_offset
-            entry.image_pos = self.image_pos + entry.offset
+            entry.SetImagePos(image_pos + self.offset)
             if entry._cbfs_compress:
                 entry.uncomp_size = cfile.memlen