sunxi: Add limit with the MMC environment

The MMC environment offset is getting very close to the end of the U-Boot
binary now. Since we want to make sure this will not overflow, add a size
limit in the board for arm64. arm32 has already that limit enforced in our
custom image generation.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Reviewed-by: Jagan Teki <jagan@openedev.com>