mb/google/mancomb: Add new mainboard

Mancomb is a new Google mainboard with an AMD Cezanne SOC.

BUG=b:175143925
TEST=builds

Change-Id: I1264f44a0b986f7f7c89ac7b42f1e4e4119a35e6
Signed-off-by: Mathew King <mathewk@chromium.org>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/50007
Reviewed-by: Felix Held <felix-coreboot@felixheld.de>
Reviewed-by: EricR Lai <ericr_lai@compal.corp-partner.google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/mainboard/google/mancomb/bootblock.c

@@ -0,0 +1,9 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+
+#include <bootblock_common.h>
+#include <baseboard/variants.h>
+
+void bootblock_mainboard_early_init(void)
+{
+	/* TODO: Perform mainboard initialization */
+}