arm64: dts: imx8mq-librem5: lower the mipi csi 1 frequencies

No frames are streamed when using the default frequencies. I'm not yet
sure why the fastest ones don't work here but we've been using these
frequencies successfully for a long time now.

Signed-off-by: Martin Kepplinger <martin.kepplinger@puri.sm>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>