mvebu: consolidate SPL boot device config symbols

Use MVEBU_SPL_BOOT_DEVICE_* to select between SPI and MMC, instead of
board specific symbols. This commit enables the boot device selection
menu to all mvebu platforms, but it is only effective on Turris Omnia
and gdsys Controlcenter DC platforms. A following commit will enable
boot selection for other platforms.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Stefan Roese <sr@denx.de>