Add i.MX6SX/UL arm2 boards support. Most code are from imx_v2015.04, but adapted to 2016.03 release.
Tested on mx6ul_14x14_ddr3_arm2 and mx6sx_19x19_ddr3_arm2.
Signed-off-by: Peng Fan
CONFIG_ARM=y CONFIG_ARCH_MX6=y CONFIG_TARGET_MX6SX_19X19_ARM2=y CONFIG_SYS_EXTRA_OPTIONS="IMX_CONFIG=board/freescale/mx6sx_19x19_arm2/imximage.cfg,SYS_BOOT_NAND" CONFIG_CMD_GPIO=y
