Add support for LTC2991 Octal I2C Voltage, Current, and Temperature
Monitor.

The LTC2991 is used to monitor system temperatures, voltages and
currents. Through the I2C serial interface, the eight monitors can
individually measure supply voltages and can be paired for
differential measurements of current sense resistors or temperature
sensing transistors. Additional measurements include internal
temperature and internal VCC.
Signed-off-by: Antoniu Miclaus <antoniu.miclaus@analog.com>
Link: https://lore.kernel.org/r/20231026103413.27800-2-antoniu.miclaus@analog.com
[groeck: Fixed up documentation warning]
Signed-off-by: Guenter Roeck <linux@roeck-us.net>