doc: board: Convert README.sh7752evb 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 f9aa5c9..339a2a1 100644
--- a/doc/board/index.rst
+++ b/doc/board/index.rst
@@ -11,4 +11,5 @@
    emulation/index
    google/index
    intel/index
+   renesas/index
    sifive/index