arm: mvebu: Implement return_to_bootrom() via U-Boot's SPL framework

U-Boot's SPL framework already has an API for loading U-Boot via
BootROM.

Implement the function board_return_to_bootrom() for mvebu SPL code.

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>