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/MAINTAINERS b/MAINTAINERS
index 92d4a15..fac9ce8 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -1281,6 +1281,13 @@
 F:	drivers/video/mali_dp.c
 F:	drivers/i2c/i2c-versatile.c
 
+MEMBUF
+M:	Simon Glass <sjg@chromium.org>
+S:	Maintained
+T:	git https://source.denx.de/u-boot/u-boot.git
+F:	include/membuf.h
+F:	lib/membuf.c
+
 MICROBLAZE
 M:	Michal Simek <monstr@monstr.eu>
 S:	Maintained