x86: dts: Mark serial as needed before relocation

We almost always need the serial port before relocation, so mark it as such.
This will ensure that it appears in the device tree for SPL, if used.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>

arch/x86/dts/serial.dtsi

@@ -1,5 +1,6 @@
 / {
 	serial: serial {
+		u-boot,dm-pre-reloc;
 		compatible = "ns16550";
 		reg = <0x3f8 8>;
 		reg-shift = <0>;