ENGR00161317 - MX6Q: Integrate plugin and dcd DRAM init script in uboot.

- Add plugin DRAM init script in flash_header.S file.
- Define CONFIG_FLASH_PLUG_IN in mx6q_sabreauto.h to switch plugin mode.
- DDR support 528MHz and 480MHz in plugin mode.
  Switch DDR clock to 480M according to define CONFIG_IPG_40M_FR_PLL3.

Signed-off-by: Fugang Duan <B38611@freescale.com>