arm: imx: imx8mn_beacon: Remove unnecessary configs

Because the Beacon imx8mn board uses device tree
for MMC/SD and Ethernet, there is no need to have some of the
config options. It's handled by the device tree.

Signed-off-by: Adam Ford <aford173@gmail.com>
Reviewed-by: Fabio Estevam <festevam@denx.de>