dts: imx28: Remove #include "imx28.dtsi" from imx28-u-boot.dtsi file

After this change it is possible to use imx28-<board>-u-boot.dtsi with
the imx28-u-boot.dtsi explicitly included without breaking setup from
imx28-<board>.dts file.

The problem is that the imx28.dtsi included in a wrong place overrides the
changes made in imx28-<board>.dts. As a result some devices are "disabled"
in the final DTB.
Signed-off-by: NLukasz Majewski <lukma@denx.de>

arch/arm/dts/imx28-u-boot.dtsi

@@ -5,7 +5,6 @@
  *
  * SPDX-License-Identifier:     GPL-2.0+ or X11
  */
-#include "imx28.dtsi"
 
 &gpio0 {
 	gpio-ranges = <&pinctrl 0 0 29>;