hwmon: (ltc2992) Add support

LTC2992 is a rail-to-rail system monitor that
measures current, voltage, and power of two supplies.

Two ADCs simultaneously measure each supply’s current.
A third ADC monitors the input voltages and four
auxiliary external voltages.

Signed-off-by: Alexandru Tachici <alexandru.tachici@analog.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>