mmc: MMC 4.4 DDR support

Add support for Dual Data Rate MMC cards as defined in the 4.4
specification.

Signed-off-by: Hanumath Prasad <hanumath.prasad@stericsson.com>
Cc: linux-mmc@vger.kernel.org
Acked-by: Linus Walleij <linus.walleij@stericsson.com>
Tested-by Zhangfei Gao <zhangfei.gao@marvell.com>
Signed-off-by: Chris Ball <cjb@laptop.org>