LF-5464: arm: dts: imx7ulp: assign parent clocks for lcdif and mipi dsi

commit e8b9961608097f3a49afede797c9ed13fca4c6ab from
https://source.codeaurora.org/external/imx/linux-imx

Currently, mipi dsi and lcdif pixel clock use default parent clocks
that setting in uboot. Drivers will fail to work when uboot setting
changed.
Assign their parent clocks to avoid such issue.

Signed-off-by: Sandor Yu <Sandor.yu@nxp.com>
Reviewed-by: Ye Li <ye.li@nxp.com>
Signed-off-by: Xiaolei Wang <xiaolei.wang@windriver.com>