google/reef: Add wake signal for trackpad

EVT has a wake signal for track pad which is routed to GP_15.

BUG=chrome-os-partner:54960

Change-Id: I9a73a3dc74e3bbed63509a3c076ec17a6559da55
Signed-off-by: Furquan Shaikh <furquan@google.com>
Reviewed-on: https://review.coreboot.org/15723
Tested-by: build bot (Jenkins)
Reviewed-by: Aaron Durbin <adurbin@chromium.org>

src/mainboard/google/reef/devicetree.cb

@@ -100,6 +100,7 @@ chip soc/intel/apollolake
 				register "hid" = ""ELAN0000""
 				register "desc" = ""ELAN Touchpad""
 				register "irq" = "IRQ_EDGE_LOW(GPIO_18_IRQ)"
+				register "wake" = "GPIO_15"
 				device i2c 15 on end
 			end
 		end # - I2C 4