arm: mvebu: Add SDIO/SDHCI support for Armada A38x

Armada A38x implements an SDHCI compatible SDIO controller. This patch
enables the Marvell driver to support this SoC. And enables the
SDIO controller if selected by the board configuration.

Tested on Marvell DB-88F6820-GP board.

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