dm: serial_pl01x: Add missing private data size

The private data size is missing from the driver, so we store it at 0,
which causes problems when something overwrites memory at 0.

Fix this.

Change-Id: I6f551ee905b0064ae8343e41e46450c37c8c8c1a
Signed-off-by: Simon Glass <sjg@chromium.org>
Tested-by: Stephen Warren <swarren@wwwdotorg.org>

drivers/serial/serial_pl01x.c

@@ -348,6 +348,7 @@ U_BOOT_DRIVER(serial_pl01x) = {
 	.probe = pl01x_serial_probe,
 	.ops	= &pl01x_serial_ops,
 	.flags = DM_FLAG_PRE_RELOC,
+	.priv_auto_alloc_size = sizeof(struct pl01x_priv),
 };
 
 #endif