mmc: sdhci-esdhc-imx: add DDR mode support for mx6

When DDR mode is enabled, the initial pre_div should be 2.
And the pre_div value should be changed accordingly
from
...
02h) Base clock divided by 4
01h) Base clock divided by 2
00h) Base clock divided by 1
to
..
02h) Base clock divided by 8
01h) Base clock divided by 4
00h) Base clock divided by 2

Signed-off-by: Dong Aisheng <b29396@freescale.com>
Acked-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Chris Ball <cjb@laptop.org>