sandbox: Add an alias for SPI

At present 'sf probe' does not work since it cannot find SPI bus 0. Add
an alias to correct this, now that we no-longer have the --spi_sf option.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/sandbox/dts/sandbox64.dts b/arch/sandbox/dts/sandbox64.dts
index a263f78..da2b442 100644
--- a/arch/sandbox/dts/sandbox64.dts
+++ b/arch/sandbox/dts/sandbox64.dts
@@ -10,6 +10,7 @@
 		pci0 = &pci;
 		rtc0 = &rtc_0;
 		axi0 = &axi;
+		spi0 = &spi;
 	};
 
 	cros_ec: cros-ec {
@@ -48,7 +49,7 @@
 				0x01000000 0 0x20000000 0 0x20000000 0 0x2000>;
 	};
 
-	spi@0 {
+	spi: spi@0 {
 		u-boot,dm-pre-reloc;
 		#address-cells = <1>;
 		#size-cells = <0>;