arm: mvebu: turris_omnia: update defconfig

Add Macronis SPI flash support - some Omnia routers are shipped with
this SPI flash chip.
Also add the memtest command to defconfig.

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