ARM: 7617/1: mmc: mmci: Support for DDR mode

Add support for DDR mode which may be used for the ux500v2 variant.
Corresponding capabilities to enable the DDR support must be set in
the platform struct to enable the functionality.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>