On some architectures certain values of splashimage will lead to
a data abort exception.

Document the problem, and implement a callback for splashimage to
reject such values.

Cc: Anatolij Gustschin <agust@denx.de>
Cc: Wolfgang Denk <wd@denx.de>
Signed-off-by: Nikita Kiryanov <nikita@compulab.co.il>
Acked-by: Igor Grinberg <grinberg@compulab.co.il>