configs: imx8mp_beacon: Fall back to using TF-A

When the board was originally added, it enabled some features which
allowed it to bypass Trusted Firmware, but as the feature set of
Linux grew and more features became available, the U-Boot config
options which bypassed TF-A caused issues, so it needs to return
to the standard operating mode of using TF-A or the system no
longer boots.

Fixes: ab53bd43 ("arm64: imx: Add support for imx8mp-beacon-kit")
Signed-off-by: Adam Ford <aford173@gmail.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>