arm: mvebu: db-88f6820-gp: Add SDIO/MMC SPL boot support

This patch adds the configuration options to boot via SDIO/MMC on the
Marvell DB-88F6820-GP Armada A38x board. The default boot device
is still SPI NOR flash.

To enable MMC booting on this board 2 things need to be changes:
a) Change kwbimage.cfg
   BOOT_FROM   sdio
b) In the config header select
   #define CONFIG_SPL_BOOT_DEVICE	SPL_BOOT_SDIO_MMC_CARD

The generated image needs to be copied to the first bootable MMC
partition:

dd if=u-boot-spl.kwb of=/dev/sdX1

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Luka Perkov <luka.perkov@sartura.hr>
Cc: Dirk Eibach <eibach@gdsys.de>