doc: board: Add Intel Bay Trail based board docs

This extracts Intel Bay Trail based board specific information from
README.x86, converts plain text documentation to reST format and
adds them to Sphinx TOC tree. No essential content change.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
diff --git a/doc/board/intel/index.rst b/doc/board/intel/index.rst
index 93b8ba2..af24760 100644
--- a/doc/board/intel/index.rst
+++ b/doc/board/intel/index.rst
@@ -6,4 +6,6 @@
 .. toctree::
    :maxdepth: 2
 
+   bayleybay
    crownbay
+   minnowmax