imx: imx8mm: imx8mm-kontron-n801x-s: add common board u-boot.dtsi

imx: imx8mm: imx8mm-kontron-n801x-s: add common board u-boot.dtsi

When using a board variant that selects the lvds specific dtb the
*.u-boot.dtsi file will not be included. To have a lvds dtb specific
u-boot.dtsi file move this part to a common board u-boot.dtsi file and
include this in the board base u-boot.dtsi and create an additional one
for the lvds variant.

Signed-off-by: Heiko Thiery <heiko.thiery@gmail.com>
Reviewed-by: Frieder Schrempf <frieder.schrempf@kontron.de>