mx6sabresd: Update the config file

Only configs/mx6sabresd_defconfig is supported now, so update the
MAINTAINERS file accordingly.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>

board/freescale/mx6sabresd/MAINTAINERS

@@ -3,6 +3,4 @@ M:	Fabio Estevam <fabio.estevam@nxp.com>
 S:	Maintained
 F:	board/freescale/mx6sabresd/
 F:	include/configs/mx6sabresd.h
-F:	configs/mx6dlsabresd_defconfig
-F:	configs/mx6qsabresd_defconfig
-F:	configs/mx6sabresd_spl_defconfig
+F:	configs/mx6sabresd_defconfig