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

This patch adds MMC/SDIO support to the Marvell DB-88F6820-GP board
configuration. Including support for the common partitions and
filesystems.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Pantelis Antoniou <panto@antoniou-consulting.com>
Cc: Luka Perkov <luka.perkov@sartura.hr>