mmc: dw_mmc: Add Synopsys DesignWare mmc host driver.

This adds the mmc host driver for the Synopsys DesignWare mmc
host controller, found in a number of embedded SoC designs.

Signed-off-by: Will Newton <will.newton@imgtec.com>
Reviewed-by: Matt Fleming <matt@console-pimps.org>
Reviewed-by: Chris Ball <cjb@laptop.org>
Signed-off-by: Chris Ball <cjb@laptop.org>