4xx: adjust TEXT_BASE to increase U-Boot image size

On some boards (canyonlands, luan, sequoia) we need more room for the
U-Boot image to allow for new features like the new environment code.
Shift TEXT_BASE as needed.

Signed-off-by: Wolfgang Denk <wd@denx.de>
Cc: Stefan Roese <sr@denx.de>
Acked-by: Stefan Roese <sr@denx.de>