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/board/engicam/imx6ul/MAINTAINERS b/board/engicam/imx6ul/MAINTAINERS
index 73dbec8..88db309 100644
--- a/board/engicam/imx6ul/MAINTAINERS
+++ b/board/engicam/imx6ul/MAINTAINERS
@@ -8,7 +8,12 @@
 F:	configs/imx6ul_isiot_emmc_defconfig
 F:	configs/imx6ul_isiot_mmc_defconfig
 F:	configs/imx6ul_isiot_nand_defconfig
+F:	arch/arm/dts/imx6ul.dtsi
+F:	arch/arm/dts/imx6ul-u-boot.dtsi
 F:	arch/arm/dts/imx6ul-geam-kit.dts
+F:	arch/arm/dts/imx6ul-geam-kit-u-boot.dtsi
 F:	arch/arm/dts/imx6ul-isiot.dtsi
+F:	arch/arm/dts/imx6ul-isiot-u-boot.dtsi
 F:	arch/arm/dts/imx6ul-isiot-emmc.dts
+F:	arch/arm/dts/imx6ul-isiot-emmc-u-boot.dtsi
 F:	arch/arm/dts/imx6ul-isiot-nand.dts