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.com


Signed-off-by: Guenter Roeck <linux@roeck-us.net>