clk: imx: Correct the root clk of media ldb on imx8mp

The root clock slice at 0xbf00 is media_ldb clock,
not csi_phy2_ref, so correct it.

Signed-off-by: Jacky Bai <ping.bai@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>