ntb: idt: Add basic hwmon sysfs interface

IDT PCIe switches provide an embedded temperature sensor working
within [0; 127.5]C with resolution of 0.5C. They also can generate
a PCIe upstream interrupt in case if the temperature passes through
specified thresholds. Since this thresholds interface is very broken
the created hwmon-sysfs interface exposes only the next set of hwmon
nodes: current input temperature, lowest and highest values measured,
history resetting, value offset. HWmon alarm interface isn't provided.

IDT PCIe switch also've got an ADC/filter settings of the sensor.
This driver doesn't expose them to the hwmon-sysfs interface at the
moment, except the offset node.

Signed-off-by: Serge Semin <fancer.lancer@gmail.com>
Signed-off-by: Jon Mason <jdmason@kudzu.us>