colibri-imx8x: remove -u-boot.dtsi include

This file is included automatically since quite some time now.

From documentation:

U-Boot automatically looks for and includes a file with updates
to the standard devicetree for your board, searching for them in
the same directory as the main file, in this order:

<orig_filename>-u-boot.dtsi
<CONFIG_SYS_SOC>-u-boot.dtsi
<CONFIG_SYS_CPU>-u-boot.dtsi
<CONFIG_SYS_VENDOR>-u-boot.dtsi
u-boot.dtsi

Signed-off-by: Andrejs Cainikovs <andrejs.cainikovs@toradex.com>