ARM: dts: i.MX6UL: U-Boot specific dts for u-boot, dm-spl

u-boot,dm-spl property is specific to U-Boot, so move it into
*u-boot.dtsi files for relevant i.MX6UL files.

This make syncing Linux dts files straight forward.

Also update the MAINTAINERS file for dts files.

Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
diff --git a/arch/arm/dts/imx6ul-isiot-nand.dts b/arch/arm/dts/imx6ul-isiot-nand.dts
index 12a3528..ffdaf34 100644
--- a/arch/arm/dts/imx6ul-isiot-nand.dts
+++ b/arch/arm/dts/imx6ul-isiot-nand.dts
@@ -42,6 +42,7 @@
 
 /dts-v1/;
 
+#include "imx6ul.dtsi"
 #include "imx6ul-isiot.dtsi"
 
 / {