hv: uart: enable early boot uart

Enable uart as early as possible to make things easier for debugging.
After this we could use printf to output information to the uart. As for
pr_xxx APIs, they start to work when init_logmsg is called.

Tracked-On: #2987
Signed-off-by: Li, Fei1 <fei1.li@intel.com>

hypervisor/release/uart16550.c

@@ -0,0 +1,9 @@
+/*
+ * Copyright (C) 2019 Intel Corporation. All rights reserved.
+ *
+ * SPDX-License-Identifier: BSD-3-Clause
+ */
+
+#include <types.h>
+
+void uart16550_init(__unused bool early_boot) {}