membuff: Rename the files to membuf

Rename the C and header files to use the membuf basename, to match the
functions.

Add a MAINTAINERS entry while we are here.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/include/asm-generic/global_data.h b/include/asm-generic/global_data.h
index 789adf2..18d9459 100644
--- a/include/asm-generic/global_data.h
+++ b/include/asm-generic/global_data.h
@@ -23,7 +23,7 @@
 #include <board_f.h>
 #include <event_internal.h>
 #include <fdtdec.h>
-#include <membuff.h>
+#include <membuf.h>
 #include <linux/list.h>
 #include <linux/build_bug.h>
 #include <asm-offsets.h>