mx6sabresd: Add mx6sabresd_spl_defconfig to MAINTAINERS entry

Let's add mx6sabresd_spl_defconfig entry into MAINTAINERS, so that we avoid
getting a warning that the mx6sabresd_spl is not maintained.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
diff --git a/board/freescale/mx6sabresd/MAINTAINERS b/board/freescale/mx6sabresd/MAINTAINERS
index 69c0a30..0011ec7 100644
--- a/board/freescale/mx6sabresd/MAINTAINERS
+++ b/board/freescale/mx6sabresd/MAINTAINERS
@@ -5,3 +5,4 @@
 F:	include/configs/mx6sabresd.h
 F:	configs/mx6dlsabresd_defconfig
 F:	configs/mx6qsabresd_defconfig
+F:	configs/mx6sabresd_spl_defconfig