arm64: dts: renesas: r8a77990: Remove invalid compatible value for CSI40

The compatible value renesas,rcar-gen3-csi2 was used while prototyping
the R-Car CSI-2 driver but was removed before the driver was merged.
Remove the only occurrence of the compatible value which manage to make
it upstream.

Fixes: ec70407a ("arm64: dts: renesas: r8a77990: Add VIN and CSI-2 device nodes")
Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>