dm: spi: Use device_bind_driver() instead of our own function

The SPI function does the same thing, so we may as well just use the new
generic function. The 'cs' parameter was not actually used, so can be
dropped.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Jagannadha Sutradharudu Teki <jagannadh.teki@gmail.com>
Acked-by: Heiko Schocher <hs@denx.de>