mmc: gen_atmel_mci: add driver model support for mci

Add the driver model support for Atmel mci while retaining the
existing legacy code. This allows the driver to support boards
that have converted to driver model as well as those that have not.

Signed-off-by: Wenyou Yang <wenyou.yang@atmel.com>
Reviewed-by: Simon Glass <sjg@chromium.org>