Add support to map devicetree into add-on (#519)

* Add support to map devicetree into add-on

* Update const.py

* Update validate.py

* Update addon.py

* Update addons.py

* Update API.md

hassio/docker/addon.py

@@ -201,6 +201,8 @@ class DockerAddon(DockerInterface):
                 }})
 
         # Init other hardware mappings
+
+        # GPIO support
         if self.addon.with_gpio:
             volumes.update({
                 "/sys/class/gpio": {
@@ -211,6 +213,14 @@ class DockerAddon(DockerInterface):
                 },
             })
 
+        # DeviceTree support
+        if self.addon.with_devicetree:
+            volumes.update({
+                "/sys/firmware/devicetree": {
+                    'bind': "/sys/firmware/devicetree", 'mode': 'r'
+                },
+            })
+
         # Host dbus system
         if self.addon.host_dbus:
             volumes.update({