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/arch/sandbox/dts/sandbox.dtsi b/arch/sandbox/dts/sandbox.dtsi
index ebc4ece..2fb365d 100644
--- a/arch/sandbox/dts/sandbox.dtsi
+++ b/arch/sandbox/dts/sandbox.dtsi
@@ -150,7 +150,7 @@
 		firmware_storage_spi: flash@0 {
 			u-boot,dm-pre-reloc;
 			reg = <0>;
-			compatible = "spansion,m25p16", "sandbox,spi-flash";
+			compatible = "spansion,m25p16", "jedec,spi-nor";
 			spi-max-frequency = <40000000>;
 			sandbox,filename = "spi.bin";
 		};