arm: mvebu: Check that kwbimage offset and blocksize are valid

There are certain restrictions for kwbimage offset and blocksize.
Validate them.

Signed-off-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Stefan Roese <sr@denx.de>