uboot-imx

imx6: make sure MMDC_CHx_MASK is clear to avoid warm reset failure

Boot ROM may mask MMDC_CHx_MASK in CCM_CCDR(such as i.MX6SL TO1.2),
it will cause warm reset fail, need to clear this MMDC_CHx_MASK field
to make sure all the i.MX6 series SOCs reset function work. Otherwise,
uboot "reset" command will fail, tested on i.MX6SL EVK board with TO1.2.

Signed-off-by: Anson Huang <b20788@freescale.com>