arm64: dts: imx8mq-librem5-r4: add the correct panel

The r4 ("Evergreen") hardware revision of the Librem 5 phone includes a
slightly different panel than the revisions before it. Since its'
description is available, describe it properly for the board.

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