mmc: Add Marvell Xenon SDHCI controller driver

This driver implementes platform specific code for the Xenon SDHCI
controller which is integrated in the Marvell MVEBU Armada 37xx and
Armada 7k / 8K SoCs.

History:
This driver is ported from the Marvell U-Boot version 2015.01 which is
written by Victor Gu <xigu@marvell.com> with minor changes ported from
the Linux driver which is written by Ziji Hu <huziji@marvell.com>.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Jaehoon Chung <jh80.chung@samsung.com>
Cc: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>