arm: mvebu: Mark return_to_bootrom() as a noreturn function

This function does not return, so add the appropriate compiler flag.

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Stefan Roese <sr@denx.de>
Reviewed-by: Chris Packham <judge.packham@gmail.com>
Tested-by: Chris Packham <judge.packham@gmail.com>