Add missing ZHA button strings (home-assistant#157335)

Author: TheJulianJES

homeassistant/components/zha/strings.json

@@ -189,6 +189,8 @@
       "button_4": "Fourth button",
       "button_5": "Fifth button",
       "button_6": "Sixth button",
+      "button_7": "Seventh button",
+      "button_8": "Eighth button",
       "close": "[%key:common::action::close%]",
       "dim_down": "Dim down",
       "dim_up": "Dim up",