abuf: Provide a way to get the buffer address

In many cases it is useful to get the address of a buffer, e.g. when
booting from it. Add a function to handle this.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/lib/abuf.c b/lib/abuf.c
index 937c3df..8156177 100644
--- a/lib/abuf.c
+++ b/lib/abuf.c
@@ -26,6 +26,12 @@
 {
 	abuf_set(abuf, map_sysmem(addr, size), size);
 }
+
+ulong abuf_addr(const struct abuf *abuf)
+{
+	return map_to_sysmem(abuf->data);
+}
+
 #else
 /* copied from lib/string.c for convenience */
 static char *memdup(const void *src, size_t len)