ARM i.MX6q: set the LDB serial clock parent to the video PLL

On i.MX6q revision 1.1 and later, set the video PLL as parent for
the LDB clock branch. On revision 1.0, the video PLL is useless
due to missing dividers, so keep the default parent (mmdc_ch1_axi).

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>