add cleanup method

Author: pantheraleo-7

mfrc522/SimpleMFRC522.py

@@ -88,3 +88,7 @@ def uid_to_num(self, uid):
       for i in range(0, 5):
           n = n * 256 + uid[i]
       return n
+
+  @staticmethod
+  def cleanup():
+      GPIO.cleanup()