imx: mx6dlarm2: Add support for i.MX6Q/DL arm2 LPDDR2 boards

Update the ddr scripts for LPDDR2 and add two build configs for LPDDR2
arm2 board. Since the LPDDR2 arm2 board has different DDR size, use
CONFIG_DDR_MB in defconfig to replace the PHYS_SDRAM_SIZE.

Signed-off-by: Ye.Li <B37916@freescale.com>
diff --git a/board/freescale/mx6qarm2/MAINTAINERS b/board/freescale/mx6qarm2/MAINTAINERS
index c8d594b1..52cf7f9 100644
--- a/board/freescale/mx6qarm2/MAINTAINERS
+++ b/board/freescale/mx6qarm2/MAINTAINERS
@@ -6,3 +6,5 @@
 F:	include/configs/mx6qarm2.h
 F:	configs/mx6qarm2_defconfig
 F:	configs/mx6dlarm2_defconfig
+F:	configs/mx6qarm2_lpddr2_defconfig
+F:	configs/mx6dlarm2_lpddr2_defconfig