dm: spi: Adjust cmd_spi to work with driver model

Driver model uses a different way to find the SPI bus and slave from the
numbered devices given on the command line. Adjust the code to suit.

We use a generic SPI device, and attach it to the SPI bus before performing
the transaction.

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