x86: Add support for private files

Some boards need to include binary data into the image for use during the
boot process. Add a node for these.

An example is the audio-codec configuration used by some audio drivers on
Intel platforms. If no private files are provided, they will be omitted.

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

arch/x86/dts/u-boot.dtsi

@@ -138,6 +138,10 @@
 		filename = CONFIG_FSP_FILE_S;
 	};
 #endif
+	private_files: private-files {
+		type = "files";
+		pattern = "*.dat";
+	};
 #ifdef CONFIG_HAVE_CMC
 	intel-cmc {
 		filename = CONFIG_CMC_FILE;