imx: kontron-sl-mx8mm: Remove LVDS board type and devicetrees

The display isn't and won't be used in U-Boot. Also the display setup
is not yet supported in mainline Linux, so even for cases where the
U-Boot devicetree is passed to the kernel there is currently no use
for this configuration.

Selecting the proper configuration in the kernel FIT image automatically
depending on the detected hardware can be handled by a script in the
environment.

Signed-off-by: Frieder Schrempf <frieder.schrempf@kontron.de>
Reviewed-by: Fabio Estevam <festevam@denx.de>