Add an MMC driver which supports RK3288, but may also support other SoCs.
It uses the Designware MMC device.
Signed-off-by: Simon Glass <sjg@chromium.org>
Add basic support for MMC, providing a uclass which can set up an MMC
device. This allows MMC drivers to move to using driver model.
Signed-off-by: Simon Glass <sjg@chromium.org>