omap3: cm-t3517: change environment size

Mainline CM-T3517 U-Boot environment size differs from that one
shipped with CM-T3517 boards.

Update environment size, to avoid backward compatibility issues.

Signed-off-by: Dmitry Lifshitz <lifshitz@compulab.co.il>
Acked-by: Igor Grinberg <grinberg@compulab.co.il>