doc: Bring in FIT x86 boot

Bring this file into the documentation.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/doc/usage/fit/index.rst b/doc/usage/fit/index.rst
index 93a9623..0635d06 100644
--- a/doc/usage/fit/index.rst
+++ b/doc/usage/fit/index.rst
@@ -11,3 +11,4 @@
     :maxdepth: 1
 
     source_file_format
+    x86-fit-boot