x86: Add a dtsi file for the pc speaker

The pc speaker driven by the i8254 is generic enough to deserve
a single dtsi file to be included by boards that use it.

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

arch/x86/dts/pcspkr.dtsi

@@ -0,0 +1,5 @@
+/ {
+	pcspkr {
+		compatible = "i8254,beeper";
+	};
+};