arm: mvebu: turris_mox: enable setexpr command in defconfig

Enable setexpr command in defconfig for Turris MOX. We have found that
this is sometimes useful in U-Boot scripts and there is enough space in
NOR memory on MOX.

Signed-off-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Stefan Roese <sr@denx.de>