hwmon: (lm90) Add flag to indicate 'alarms' attribute support

We don't want to support the obsolete 'alarms' attribute for new
chips supported by this driver. Add flag to indicate 'alarms' attribute
support and use it for existing chips. This flag will not be set for
additional chips supported by this driver in the future.

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