board: toradex: verdin-imx8mm: add README

Add README with build steps for U-boot and TF-A for Verdin iMX8M Mini SoM.

Signed-off-by: Igor Opaniuk <igor.opaniuk@toradex.com>
Signed-off-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Reviewed-by: Oleksandr Suvorov <oleksandr.suvorov@toradex.com>
1 file changed