board/freescale: Update MAINTAINERS files

Update maintainers for secure boot targets.

Signed-off-by: York Sun <york.sun@nxp.com>
diff --git a/board/freescale/ls1021atwr/MAINTAINERS b/board/freescale/ls1021atwr/MAINTAINERS
index b997bb0..06d888f 100644
--- a/board/freescale/ls1021atwr/MAINTAINERS
+++ b/board/freescale/ls1021atwr/MAINTAINERS
@@ -9,3 +9,7 @@
 F:	configs/ls1021atwr_sdcard_ifc_defconfig
 F:	configs/ls1021atwr_sdcard_qspi_defconfig
 F:	configs/ls1021atwr_qspi_defconfig
+
+M:	Sumit Garg <sumit.garg@nxp.com>
+S:	Maintained
+F:	configs/ls1021atwr_sdcard_ifc_SECURE_BOOT_defconfig