arm: spear: Enable THUMB mode on x600 board

To reduce the size of the U-Boot image on the x600 board, lets enable
the THUMB mode. This reduces the overall size to less than 0x6000
bytes. Fitting it again in the onboard NOR flash.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Viresh Kumar <viresh.kumar@linaro.org>