board: tbs2910: Remove FIT support in defconfig to reduce u-boot size

The current defconfig build generates a u-boot.imx file that is too large
for the available space on a eMMC/SD card. Installing this file overwrites
the u-boot environment. So disable the unused FIT support to reduce the
size of the u-boot binary.

Signed-off-by: Soeren Moch <smoch@web.de>