sandbox: Drop ad-hoc device declarations in SPL

Since sandbox's SPL is build with of-platadata, we should not use
U_BOOT_DEVICE() declarations as well. Drop them.

Signed-off-by: Simon Glass <sjg@chromium.org>

board/sandbox/sandbox.c

@@ -21,10 +21,12 @@
  */
 gd_t *gd;
 
+#if !CONFIG_IS_ENABLED(OF_PLATDATA)
 /* Add a simple GPIO device */
 U_BOOT_DEVICE(gpio_sandbox) = {
 	.name = "sandbox_gpio",
 };
+#endif
 
 void flush_cache(unsigned long start, unsigned long size)
 {