mb/google/myst: Add variant makefile

Add variant makefile to support including the memory folder for Myst.

BUG=b:273383819
TEST=Builds in chromium with blobs

Change-Id: I03b0cd91dd66f357b15522da36f5118867b6b14c
Signed-off-by: Jon Murphy <jpmurphy@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/74964
Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Karthik Ramasubramanian <kramasub@google.com>
Reviewed-by: Raul Rangel <rrangel@chromium.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/mainboard/google/myst/variants/myst/Makefile.inc

@@ -0,0 +1,3 @@
+# SPDX-License-Identifier: GPL-2.0-or-later
+
+subdirs-y += ./memory