uboot-imx

dm: spi: Add documentation on how to convert over SPI drivers

This README is intended to help maintainers move their SPI drivers over to
driver model. It works through the required steps with an example.

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