dm: sf: Add tests for SPI flash

Add a simple test for SPI that uses SPI flash. It operates by creating a
SPI flash file and using the 'sf test' command to test that all
operations work correctly.

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