ARM i.MX6: fix ldb_di_sel mux

This adds the mmdc_ch1 as a possible parent for the ldb_di clk.
According to the datasheet, this clock can be selected at this mux.

Signed-off-by: Steffen Trumtrar <s.trumtrar@pengutronix.de>
Acked-by: Shawn Guo <shawn.guo@linaro.org>