iio: Documentation: Add max9611 sysfs documentation

Add documentation for max9611 driver.
Document attributes describing value of shunt resistor installed between
RS+ and RS- voltage sense inputs.

Signed-off-by: Jacopo Mondi <[e-mail]>
Signed-off-by: Jonathan Cameron <[e-mail]>

Documentation/ABI/testing/sysfs-bus-iio-adc-max9611

···
+What:		/sys/bus/iio/devices/iio:deviceX/in_power_shunt_resistor
+Date:		March 2017
+KernelVersion:	4.12
+Contact:	[e-mail]
+Description: 	The value of the shunt resistor used to compute power drain on
+                common input voltage pin (RS+). In Ohms.
+
+What:		/sys/bus/iio/devices/iio:deviceX/in_current_shunt_resistor
+Date:		March 2017
+KernelVersion:	4.12
+Contact:	[e-mail]
+Description: 	The value of the shunt resistor used to compute current flowing
+                between RS+ and RS- voltage sense inputs. In Ohms.
+
+These attributes describe a single physical component, exposed as two distinct
+attributes as it is used to calculate two different values: power load and
+current flowing between RS+ and RS- inputs.