dm: exynos: Convert SPI to driver model

Move the exynos SPI driver over to driver model. This removes quite a bit
of boilerplate from the driver, although it adds some for driver model.

A few device tree additions are needed to make the SPI flash available.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Jagannadha Sutradharudu Teki <jagannadh.teki@gmail.com>