imx8m: imx8mm-beacon: Migrate README to rst

Since U-Boot builds HTML documentation, migrate the contents
of the README file to an rst file which can generate the
proper outputs.

Signed-off-by: Adam Ford <aford173@gmail.com>
diff --git a/doc/board/beacon/index.rst b/doc/board/beacon/index.rst
index 1fe1046..8eba5e8 100644
--- a/doc/board/beacon/index.rst
+++ b/doc/board/beacon/index.rst
@@ -7,3 +7,4 @@
    :maxdepth: 2
 
    beacon-imx8mp
+   beacon-imx8mm