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: NFabio Estevam <fabio.estevam@freescale.com>

board/freescale/mx6sabresd/MAINTAINERS

@@ -5,3 +5,4 @@ F:	board/freescale/mx6sabresd/
 F:	include/configs/mx6sabresd.h
 F:	configs/mx6dlsabresd_defconfig
 F:	configs/mx6qsabresd_defconfig
+F:	configs/mx6sabresd_spl_defconfig