sandbox: Correct spi flash operation

Since the SPI nor conversion, 'sf probe' does not work on sandbox. Fix
this by using the expected compatible string in the flash node.

Signed-off-by: Simon Glass <sjg@chromium.org>
Fixes: cd35365762 (mtd: sf_probe: remove spi-flash compatible)
diff --git a/drivers/spi/Kconfig b/drivers/spi/Kconfig
index f9b2823..8e1424b 100644
--- a/drivers/spi/Kconfig
+++ b/drivers/spi/Kconfig
@@ -226,7 +226,7 @@
 		cs-gpios = <0>, <&gpio_a 0>;
 		flash@0 {
 			reg = <0>;
-			compatible = "spansion,m25p16", "sandbox,spi-flash";
+			compatible = "spansion,m25p16", "jedec,spi-nor";
 			spi-max-frequency = <40000000>;
 			sandbox,filename = "spi.bin";
 		};