hwmon: Add notification support

For hwmon drivers using the hwmon_device_register_with_info() API, it
is desirable to have a generic notification mechanism available. This
mechanism can be used to notify userspace as well as the thermal
subsystem if the driver experiences any events, such as warning or
critical alarms.

Implement hwmon_notify_event() to provide this mechanism. The function
generates a sysfs event and a udev event. If the device is registered
with the thermal subsystem and the event is associated with a temperature
sensor, also notify the thermal subsystem that a thermal event occurred.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Serge Semin <Sergey.Semin@baikalelectronics.ru>
Cc: Maxim Kaurkin <Maxim.Kaurkin@baikalelectronics.ru>
Cc: Alexey Malahov <Alexey.Malahov@baikalelectronics.ru>
Cc: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: linux-mips@vger.kernel.org
Cc: devicetree@vger.kernel.org
Signed-off-by: Guenter Roeck <linux@roeck-us.net>