hwmon: Add LTC2990 sensor driver

This adds support for the Linear Technology LTC2990  I2C System Monitor.
The LTC2990 supports a combination of voltage, current and temperature
monitoring. This driver currently only supports reading two currents
by measuring two differential voltages across series resistors, in
addition to the Vcc supply voltage and internal temperature.

This is sufficient to support the Topic Miami SOM which uses this chip
to monitor the currents flowing into the FPGA and the CPU parts.

Signed-off-by: Mike Looijmans <mike.looijmans@topic.nl>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>