hwmon: (ltc2978) Add support for LTC2980 and LTM2987

LTC2980 and LTM2987 are command compatible to LTC2977. They consist of
two LTC2977 on a single die, and are instantiated as two separate chips,
each supporting eight channels.

Suggested-by: Michael Jones <mike@proclivis.com>
Tested-by: Michael Jones <mike@proclivis.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>