hwmon: (adm1275) Allow setting shunt reg value

The ADM series of hotswap controllers support extending
the current measurement range by using a sensing resistor
value other than the typical 1 mOhm. For example, using a 0.5 mOhm
sensing resistor doubles the maximal current can be measured.

Current driver assumes a shunt resistor value of 1 mOhm in calculation,
meaning for other resistor values, hwmon will report scaled
current/power measurements. This patch parses device tree parameter
"shunt-resistor-micro-ohms", if there is one.

Signed-off-by: Kun Yi <kunyi@google.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>