arm64: dts: imx8mn: Fix video clock parents

There are a few clocks whose parents are set in mipi_dsi
and mxsfb nodes, but these clocks are used by the disp_blk_ctrl
power domain which may cause an issue when re-parenting, resuling
in a disp_pixel clock having the wrong parent and wrong rate.

Fix this by moving the assigned-clock-parents as associate clock
assignments to the power-domain node to setup these clocks before
they are enabled.

Fixes: d825fb64

 ("arm64: dts: imx8mn: Add display pipeline components")
Signed-off-by: Adam Ford <aford173@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>