arm64: dts: imx8mp: correct clock of pgc_ispdwp

The deprecated DIV clk is previously part of the ISP composite clk, but
there is still one child clk(IMX8MP_CLK_MEDIA_ISP_ROOT) sourcing from
IMX8MP_CLK_MEDIA_ISP( previously IMX8MP_CLK_MEDIA_ISP_DIV)

So IMX8MP_CLK_MEDIA_ISP_ROOT should be used, not IMX8MP_CLK_MEDIA_ISP_DIV.

Fixes: 9d89189d ("arm64: dts: imx8mp: Add MEDIAMIX power domains")
Signed-off-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Tested-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>