dts: am33xx: add u-boot, dm-spl to ocp bus

This is needed for having access to the devices below this bus, most
important is uart and boot-device (spi, mmc, ...) in SPL stage.

Signed-off-by: Hannes Schmelzer <oe5hpm@oevsv.at>

arch/arm/dts/am33xx.dtsi

@@ -91,6 +91,7 @@
 	 * the whole bus hierarchy.
 	 */
 	ocp {
+		u-boot,dm-spl;
 		compatible = "simple-bus";
 		#address-cells = <1>;
 		#size-cells = <1>;