ARM: dts: imx6qp: correct LDB clock inputs

On i.MX6QP the LDB clock tree has changed to move the clk gate
before the divider, to prevent clock glitches propagating downstream.

A consequence of this change is that the clk divider is now the
parent of the LDB inputs. Reflect this change in the devicetree
to allow the LDB driver to properly configure the display clocks.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>