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>
diff --git a/drivers/serial/sandbox.c b/drivers/serial/sandbox.c
index f09d291..db2fbac 100644
--- a/drivers/serial/sandbox.c
+++ b/drivers/serial/sandbox.c
@@ -267,6 +267,7 @@
 	.flags = DM_FLAG_PRE_RELOC,
 };
 
+#if !CONFIG_IS_ENABLED(OF_PLATDATA)
 static const struct sandbox_serial_platdata platdata_non_fdt = {
 	.colour = -1,
 };
@@ -275,4 +276,6 @@
 	.name = "sandbox_serial",
 	.platdata = &platdata_non_fdt,
 };
+#endif
+
 #endif /* CONFIG_IS_ENABLED(OF_CONTROL) */