hwmon: (ina238) Add support for INA780

hwmon: (ina238) Add support for INA780

INA780 is similar to the other chips in the series, but does not
support the shunt voltage register. Shunt voltage limit registers
have been renamed to current limit registers, but are otherwise
identical.

While the chip does not directly report the shunt voltage, report
it anyway by calculating its value from the current register.

Cc: Chris Packham <chris.packham@alliedtelesis.co.nz>
Reviewed-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Tested-by: Chris Packham <chris.packham@alliedtelesis.co.nz> # INA780
Signed-off-by: Guenter Roeck <linux@roeck-us.net>