uboot-imx

imx: mx6: fix mmdc ch0 clk for 6SL

>From RM, per_periph2_clk_sel option3 is:
"derive clock from 198MHz clock (divided 392MHz PLL2 PFD)."

So fix it.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Cc: Stefano Babic <sbabic@denx.de>