doc: board: Convert i.MX6 Sabresd README to reST

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

Signed-off-by: Peng Fan <peng.fan@nxp.com>
diff --git a/doc/board/freescale/index.rst b/doc/board/freescale/index.rst
index cc079ca..716bb1e 100644
--- a/doc/board/freescale/index.rst
+++ b/doc/board/freescale/index.rst
@@ -15,3 +15,4 @@
    imxrt1020-evk
    imxrt1050-evk
    mx6sabreauto
+   mx6sabresd