arm: mvebu: Move get_boot_device() to cpu.c and make visible

Move the function get_boot_device() from spl.c to cpu.c.

Make it visible, so that it may be used from other files.

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