mb/google/brya: Add entry stubs of each stage

Add entry point stubs of each stage for Brya. More functionalities will
be added later.

BUG=b:174266035
TEST=Build Test

Signed-off-by: Eric Lai <ericr_lai@compal.corp-partner.google.com>
Change-Id: I44934c05ee32090b6e34648ee02f004c83e93d57
Reviewed-on: https://review.coreboot.org/c/coreboot/+/48063
Reviewed-by: Tim Wawrzynczak <twawrzynczak@chromium.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
diff --git a/src/mainboard/google/brya/mainboard.c b/src/mainboard/google/brya/mainboard.c
new file mode 100644
index 0000000..3dc2c41
--- /dev/null
+++ b/src/mainboard/google/brya/mainboard.c
@@ -0,0 +1,18 @@
+/* SPDX-License-Identifier: GPL-2.0-or-later */
+
+#include <device/device.h>
+
+static void mainboard_init(void *chip_info)
+{
+	/* TODO: Perform mainboard initialization */
+}
+
+static void mainboard_enable(struct device *dev)
+{
+	/* TODO: Enable mainboard */
+}
+
+struct chip_operations mainboard_ops = {
+	.init = mainboard_init,
+	.enable_dev = mainboard_enable,
+};