cm_t35: prevent splashimage from being set to a bad value

Define CONFIG_SPLASHIMAGE_GUARD to prevent splashimage from being
set to a value that will cause U-Boot to hang while displaying a
splash screen.

Signed-off-by: Nikita Kiryanov <nikita@compulab.co.il>
Acked-by: Igor Grinberg <grinberg@compulab.co.il>