doc: board: verdin-imx8mm: convert readme to reST

Convert README to reStructuredText format.

Signed-off-by: Igor Opaniuk <igor.opaniuk@toradex.com>
Reviewed-by: Oleksandr Suvorov <oleksandr.suvorov@toradex.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>
[bmeng: spell out U-Boot correctly]
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
diff --git a/doc/board/toradex/index.rst b/doc/board/toradex/index.rst
index aa418d6..6cd2ade 100644
--- a/doc/board/toradex/index.rst
+++ b/doc/board/toradex/index.rst
@@ -7,3 +7,4 @@
    :maxdepth: 2
 
    colibri_imx7
+   verdin-imx8mm