arm: dts: k3-am642-r5-evm: Mark memory with u-boot, dm-spl

Mark the memory node with u-boot,dm-spl so we can use it from early SPL.
Signed-off-by: NDave Gerlach <d-gerlach@ti.com>

arch/arm/dts/k3-am642-r5-evm.dts

@@ -25,6 +25,7 @@
 		/* 2G RAM */
 		reg = <0x00000000 0x80000000 0x00000000 0x80000000>;
 
+		u-boot,dm-spl;
 	};
 
 	a53_0: a53@0 {