asus/p2b*: Move serial init into mainboard bootblock

With this bootblock messages are transmitted over serial too.

TEST=Serial messages transmitted normally on asus/p2b-ls.

Change-Id: I6f3ee68e7c76a8c6db6d75956e6a7fb75ef83850
Signed-off-by: Keith Hui <buurin@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/38670
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
diff --git a/src/mainboard/asus/p2b/bootblock.c b/src/mainboard/asus/p2b/bootblock.c
new file mode 100644
index 0000000..18eff07
--- /dev/null
+++ b/src/mainboard/asus/p2b/bootblock.c
@@ -0,0 +1,13 @@
+/* SPDX-License-Identifier: GPL-2.0-or-later */
+/* This file is part of the coreboot project. */
+
+#include <bootblock_common.h>
+#include <superio/winbond/common/winbond.h>
+#include <superio/winbond/w83977tf/w83977tf.h>
+
+#define SERIAL_DEV PNP_DEV(0x3f0, W83977TF_SP1)
+
+void bootblock_mainboard_early_init(void)
+{
+	winbond_enable_serial(SERIAL_DEV, CONFIG_TTYS0_BASE);
+}