at91sam9rlek: add missing files to MAINTAINERS

This fixes following genboardscfg.py warning:

---8<---
WARNING: no status for 'at91sam9rlek_mmc'
WARNING: no maintainers for 'at91sam9rlek_mmc'
--->8---

Signed-off-by: Andreas Bießmann <andreas.devel@googlemail.com>
diff --git a/board/atmel/at91sam9rlek/MAINTAINERS b/board/atmel/at91sam9rlek/MAINTAINERS
index 7dd8ef7..ca8c0b1 100644
--- a/board/atmel/at91sam9rlek/MAINTAINERS
+++ b/board/atmel/at91sam9rlek/MAINTAINERS
@@ -5,3 +5,4 @@
 F:	include/configs/at91sam9rlek.h
 F:	configs/at91sam9rlek_dataflash_defconfig
 F:	configs/at91sam9rlek_nandflash_defconfig
+F:	configs/at91sam9rlek_mmc_defconfig