hwmon: (ltc4151) Make shunt-resistor configurable

Allow to specify the resistance of the attached shunt via DT by
adding the shunt-resistor property. Fall-back to the previous
default (1 mOhm) if unset.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
[groeck: Fixed 'line over 80 columns' checkpatch warning]
Signed-off-by: Guenter Roeck <linux@roeck-us.net>