doc: board: Convert README.at91 to reST

Convert plain text documentation to reStructuredText format and add
it to Sphinx TOC tree. No essential content change.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
diff --git a/doc/board/index.rst b/doc/board/index.rst
index 339a2a1..b2f83b6 100644
--- a/doc/board/index.rst
+++ b/doc/board/index.rst
@@ -7,6 +7,7 @@
    :maxdepth: 2
 
    AndesTech/index
+   atmel/index
    coreboot/index
    emulation/index
    google/index