arm: mvebu: turris_omnia: Use thumb instructions in SPL to save space

With the latest changes to the drivers (SPI_FLASH_SPANSION etc), Travis
reports that the SPL image is too big. Let's use the thumb instructions
in SPL to save some space and make the image fit again.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Baruch Siach <baruch@tkos.co.il>
Cc: Marek Behun <marek.behun@nic.cz>
Reviewed-by: Marek Behún <marek.behun@nic.cz>