hwmon: (asus-ec-sensors) add doc entry for PRIME X470-PRO

Add PRIME X470-PRO to the list of supported boards.
Signed-off-by: Eugene Shalygin <eugene.shalygin@gmail.com>
Link: https://lore.kernel.org/r/20220427180237.1475954-1-eugene.shalygin@gmail.comSigned-off-by: Guenter Roeck <linux@roeck-us.net>

Documentation/hwmon/asus_ec_sensors.rst

@@ -4,6 +4,7 @@ Kernel driver asus_ec_sensors
 =================================
 
 Supported boards:
+ * PRIME X470-PRO,
  * PRIME X570-PRO,
  * Pro WS X570-ACE,
  * ProArt X570-CREATOR WIFI