Convert CONFIG_FIXED_SDHCI_ALIGNED_BUFFER to Kconfig

CONFIG_FIXED_SDHCI_ALIGNED_BUFFER is needed on some Marvell SoCs when
booting from MMC. All existing usages of this have the same value so
make this the default and have the Kconfig option depend on SPL &&
MVEBU_SPL_BOOT_DEVICE_MMC.

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