arm: mvebu: Fix description of MVEBU_SPL_BOOT_DEVICE_(SPI|MMC) options
MVEBU_SPL_BOOT_DEVICE_SPI is for NOR flash. Either serial or parallel. Not
for general serial/SPI devices. The correct name should be BOOT_DEVICE_NOR
but name SPI is already used in mkimage config format which we do not want
to change for compatibility reasons.
MVEBU_SPL_BOOT_DEVICE_MMC is for MMC and SD compatible devices. Not for
SDIO devices. In most cases used for eMMC or SD card.