hwmon: (ltc2945) Allow setting shunt resistor

Add the ability to specify the value of the shunt resistor in the
device tree instead of assuming it is 1 milliOhm. The value in the
device tree has the name shunt-resistor-micro-ohms and the
default value is 1000 micro-ohms in order to preserve the
current behavior.

Signed-off-by: Jonathan Cormier <jcormier@criticallink.com>
Signed-off-by: John Pruitt <jpruitt@criticallink.com>
[groeck: Fixed multi-line alignment, squashed last patch of series]
Signed-off-by: Guenter Roeck <linux@roeck-us.net>