arm: mvebu: Use U-Boot's SPL BootROM framework for booting from NAND/UART

arm: mvebu: Use U-Boot's SPL BootROM framework for booting from NAND/UART

Current code uses hack in board_init_f() which calls return_to_bootrom()
to skip U-Boot SPL code and return back to BootROM to load U-Boot via
UART or from NAND.

This change migrates that hack from the board_init_f() function and
changes it to return BOOT_DEVICE_BOOTROM instead of returning to BootROM
directly, so that U-Boot's SPL framework is used for returning to
BootROM.

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>