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>