riscv: dts: lichee-module-4a: Preserve memory node for SPL

Memory node is necessary for TH1520 SPL to configure size and base
address of DRAM. Let's preserve it in SPL devicetree blob.

Signed-off-by: Yao Zi <ziyao@disroot.org>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>

arch/riscv/dts/th1520-lichee-module-4a.dtsi

@@ -14,6 +14,7 @@
 	memory@0 {
 		device_type = "memory";
 		reg = <0x0 0x00000000 0x2 0x00000000>;
+		bootph-pre-ram;
 	};
 };