mainboard/google/reef: move devicetree to baseboard

Move the current devicetree.cb to be under variants/baseboard.
New variants can provide their own devicetree as needed.

BUG=chrome-os-partner:56677

Change-Id: Ib109ca4be883884b318264500d14aa8d40e3072a
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: https://review.coreboot.org/16510
Tested-by: build bot (Jenkins)
Reviewed-by: Furquan Shaikh <furquan@google.com>

src/mainboard/google/reef/Kconfig

@@ -44,6 +44,10 @@ config VARIANT_DIR
 	string
 	default "reef" if BOARD_GOOGLE_REEF
 
+config DEVICETREE
+	string
+	default "variants/baseboard/devicetree.cb"
+
 config MAINBOARD_PART_NUMBER
 	string
 	default "Reef" if BOARD_GOOGLE_REEF