[POWERPC] Fix early btext debug on PowerMac

The early btext debug wouldn't work on PowerMac when booted from BootX
due to the code looking for the wrong property name.
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>

arch/powerpc/kernel/btext.c

@@ -186,7 +186,9 @@ int btext_initialize(struct device_node *np)
 		pitch = *prop;
 	if (pitch == 1)
 		pitch = 0x1000;
-	prop = of_get_property(np, "address", NULL);
+	prop = of_get_property(np, "linux,bootx-addr", NULL);
+	if (prop == NULL)
+		prop = of_get_property(np, "address", NULL);
 	if (prop)
 		address = *prop;