Add a driver for the AM335x bandgap sensor, an on-die temperature sensor
as part of the AM335x control module extension.

TI says that the bandgap sensor is not very accurate on AM335x, but in our
tests it seems to be a good reference for the SoC temperature.

TI details:
http://processors.wiki.ti.com/index.php/AM335x_Thermal_Considerations#Measuring_Case_Temperature

Sponsored by: Rubicon Communications, LLC (Netgate)