dm: at91: Add driver model support for the spi driver

Add driver model support 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>
Acked-by: Stephen Warren <swarren@nvidia.com>