Add support for Air Purifier 4 Pro (#1287)

* Add support for Air Purifier 4 Pro

* merge airpurifier mb4 and va2 into a common interface

Author: ymj0424

README.rst

@@ -103,7 +103,7 @@ Supported devices
 -  Xiaomi Mi Robot Vacuum V1, S4, S4 MAX, S5, S5 MAX, S6 Pure, M1S, S7
 -  Xiaomi Mi Home Air Conditioner Companion
 -  Xiaomi Mi Smart Air Conditioner A (xiaomi.aircondition.mc1, mc2, mc4, mc5)
--  Xiaomi Mi Air Purifier 2, 3H, 3C, Pro, Pro H (zhimi.airpurifier.m2, mb3, mb4, v7, vb2)
+-  Xiaomi Mi Air Purifier 2, 3H, 3C, Pro, Pro H, 4 Pro (zhimi.airpurifier.m2, mb3, mb4, v7, vb2, va2)
 -  Xiaomi Mi Air (Purifier) Dog X3, X5, X7SM (airdog.airpurifier.x3, airdog.airpurifier.x5, airdog.airpurifier.x7sm)
 -  Xiaomi Mi Air Humidifier
 -  Xiaomi Aqara Camera