arm: mvebu: Remove CONFIG_SPL_BOOT_DEVICE

CONFIG_SPL_BOOT_DEVICE was made obsolete by
CONFIG_MVEBU_SPL_BOOT_DEVICE_{SPI,MMC,SATA,UART}.
CONFIG_MVEBU_SPL_BOOT_DEVICE_SPI is the default so existing users of
CONFIG_SPL_BOOT_DEVICE can simply have the option removed.

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