arm: mvebu: turris_{omnia, mox}: enable MTD command

Now that the MTD subsystem properly supports OF partitions of a SPI NOR,
we can enable the MTD command and start using it instead of the
deprecated sf command.

Signed-off-by: Marek Behún <marek.behun@nic.cz>
Cc: Stefan Roese <sr@denx.de>
Cc: Pali Rohár <pali@kernel.org>
Reviewed-by: Stefan Roese <sr@denx.de>