volteer: Create halvor variant

Create the halvor variant of the volteer reference board by copying the
template files to a new directory named for the variant.

BUG=b:151399850
BRANCH=None
TEST=util/abuild/abuild -p none -t google/volteer -x -a
make sure the build includes GOOGLE_HALVOR

Signed-off-by: Frank Wu <frank_wu@compal.corp-partner.google.com>
Change-Id: If4d3417ba55d56af441c99d949a196328d7a1951
Reviewed-on: https://review.coreboot.org/c/coreboot/+/39667
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Caveh Jalali <caveh@chromium.org>
Reviewed-by: EricR Lai <ericr_lai@compal.corp-partner.google.com>
Reviewed-by: Paul Fagerburg <pfagerburg@chromium.org>
Reviewed-by: Nick Vaccaro <nvaccaro@google.com>
diff --git a/src/mainboard/google/volteer/variants/halvor/overridetree.cb b/src/mainboard/google/volteer/variants/halvor/overridetree.cb
new file mode 100644
index 0000000..75422d8
--- /dev/null
+++ b/src/mainboard/google/volteer/variants/halvor/overridetree.cb
@@ -0,0 +1,4 @@
+chip soc/intel/tigerlake
+	device domain 0 on
+	end
+end