mmc: atmel_sdhci: Convert to the driver model support

Convert the driver to the driver model 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>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>
Reviewed-by: Heiko Schocher <hs@denx.de>