ec/google/wilco: Add wake pin configuration

Add a way for the mainboard to provide a wake pin that the EC
will use to wake the system.  This defines a _PRW object.

Change-Id: I94954104bbb8226683c37abc8c0465fe3c62a693
Signed-off-by: Duncan Laurie <dlaurie@google.com>
Reviewed-on: https://review.coreboot.org/29408
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-by: Furquan Shaikh <furquan@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/ec/google/wilco/acpi/ec_dev.asl

@@ -18,6 +18,9 @@ Device (WLCO)
 	Name (_HID, "GOOG000C")
 	Name (_UID, 1)
 	Name (_DDN, "Wilco EC Command Device")
+#ifdef EC_WAKE_PIN
+	Name (_PRW, Package () { EC_WAKE_PIN, 0x5 })
+#endif
 
 	Method (_STA)
 	{