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/configs/mx6qarm2_defconfig b/configs/mx6qarm2_defconfig
index a251615..3fe3559 100644
--- a/configs/mx6qarm2_defconfig
+++ b/configs/mx6qarm2_defconfig
@@ -1,3 +1,3 @@
-CONFIG_SYS_EXTRA_OPTIONS="IMX_CONFIG=board/freescale/mx6qarm2/imximage.cfg,MX6Q"
+CONFIG_SYS_EXTRA_OPTIONS="IMX_CONFIG=board/freescale/mx6qarm2/imximage.cfg,MX6Q,DDR_MB=2048"
 CONFIG_ARM=y
 CONFIG_TARGET_MX6QARM2=y