Add sonoff devices

The S20, TH, Basic and SV all share the same 1-channel pinout.
The 4CH has a few more IO pins.

fs/sonoff-basic.json

@@ -0,0 +1,7 @@
+{
+  "name": "Sonoff Basic, S20, SV, RF, TH, Touch",
+  "status_led": 13,
+  "channels": [
+    { "led": -1, "relay": 12, "button": 0  }
+  ]
+}