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/test/lib/abuf.c b/test/lib/abuf.c
index 7c0481a..5d61f92 100644
--- a/test/lib/abuf.c
+++ b/test/lib/abuf.c
@@ -46,7 +46,7 @@
 }
 LIB_TEST(lib_test_abuf_set, 0);
 
-/* Test abuf_map_sysmem() */
+/* Test abuf_map_sysmem() and abuf_addr() */
 static int lib_test_abuf_map_sysmem(struct unit_test_state *uts)
 {
 	struct abuf buf;
@@ -60,6 +60,8 @@
 	ut_asserteq(TEST_DATA_LEN, buf.size);
 	ut_asserteq(false, buf.alloced);
 
+	ut_asserteq(addr, abuf_addr(&buf));
+
 	return 0;
 }
 LIB_TEST(lib_test_abuf_map_sysmem, 0);