riscv: dts: hifive-unleashed-a00: Make memory node available to SPL

Make memory node available to SPL in prepration to updates to SiFive
DDR RAM driver to read memory information from DT.
Signed-off-by: NBin Meng <bin.meng@windriver.com>
Reviewed-by: NPragnesh Patel <pragnesh.patel@sifive.com>

arch/riscv/dts/hifive-unleashed-a00-u-boot.dtsi

@@ -20,6 +20,10 @@
 		u-boot,spl-payload-offset = <0x105000>; /* loader2 @1044KB */
 	};
 
+	memory@80000000 {
+		u-boot,dm-spl;
+	};
+
 	hfclk {
 		u-boot,dm-spl;
 	};