Preview 6 Release

This is the preview 6 release of dotnet/iot. Changes included this release are:

• Removed reference assembly from System.Device.Gpio package
• Added a rid-less configuration to the System.Device.Gpio package which throws PlatformNotSupported Exception.
• [Breaking Change] Added new members to IGpioController interface.
• Added support to use boards with multiple GPIO banks with libgpiod driver.
• Added support for reading an offset when using the sysfsdriver in arm64 where the pin numbering is different.
• Fixed issue with eventing on the sysfsdriver and raspberrypi driver where we would get an ObjectDisposedException when the CancellationTokenSource got disposed too soon.
• Added WriteRead method to I2CDevice which is used when atomic operations are needed to both write data and to immediately read data on the bus.
• Added IPwmController interface.
• Added support for several new bindings into the IoT.Device.Bindings package.

Packages pushed to NuGet.org:

• https://www.nuget.org/packages/System.Device.Gpio/0.1.0-prerelease.19310.5
• https://www.nuget.org/packages/Iot.Device.Bindings/0.1.0-prerelease.19310.5