arm: mvebu: armada-xp-gp.dts: Add spi0 alias

For correct spi bus detection the spi0 alias is needed in the DT.
Otherwise this error will ocurr in U-Boot:

Invalid bus 0 (err=-19)
Failed to initialize SPI flash at 0:0 (error -19)
Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Dennis Gilmore <dgilmore@redhat.com>
Tested-by: Dennis Gilmore <dgilmore@redhat.com>

arch/arm/dts/armada-xp-gp.dts

@@ -31,6 +31,10 @@
 		stdout-path = "serial0:115200n8";
 	};
 
+	aliases {
+		spi0 = &spi0;
+	};
+
 	memory@0 {
 		device_type = "memory";
 		/*