dm: sandbox: spi: Move to driver model

Adjust the sandbox SPI driver to support driver model and move sandbox over
to driver model for SPI.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Jagannadha Sutradharudu Teki <jagannadh.teki@gmail.com>
diff --git a/arch/sandbox/include/asm/state.h b/arch/sandbox/include/asm/state.h
index d17a82e..4e0981a 100644
--- a/arch/sandbox/include/asm/state.h
+++ b/arch/sandbox/include/asm/state.h
@@ -43,6 +43,7 @@
 struct sandbox_spi_info {
 	const char *spec;
 	const struct sandbox_spi_emu_ops *ops;
+	struct udevice *emul;
 };
 
 /* The complete state of the test system */