Added PwmController interface (#456)

src/System.Device.Gpio/System/Device/Pwm/IPwmController.cs

+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+namespace System.Device.Pwm
+{
+    public interface IPwmController : IDisposable
+    {
+        /// <summary>
+        /// Opens a channel in order for it to be ready to use.
+        /// </summary>
+        /// <param name="pwmChip">The PWM chip.</param>
+        /// <param name="pwmChannel">The PWM channel.</param>
+        void OpenChannel(int pwmChip, int pwmChannel);
+
+        /// <summary>
+        /// Closes an open channel.
+        /// </summary>
+        /// <param name="pwmChip">The PWM chip.</param>
+        /// <param name="pwmChannel">The PWM channel.</param>
+        void CloseChannel(int pwmChip, int pwmChannel);
+
+        /// <summary>
+        /// Changes the duty cycle for an open channel.
+        /// </summary>
+        /// <param name="pwmChip">The PWM chip.</param>
+        /// <param name="pwmChannel">The PWM channel.</param>
+        /// <param name="dutyCyclePercentage">The duty cycle percentage to change.</param>
+        void ChangeDutyCycle(int pwmChip, int pwmChannel, double dutyCyclePercentage);
+
+        /// <summary>
+        /// Starts writing to an open channel.
+        /// </summary>
+        /// <param name="pwmChip">The PWM chip.</param>
+        /// <param name="pwmChannel">The PWM channel.</param>
+        /// <param name="frequencyInHertz">The frequency in hertz to write.</param>
+        /// <param name="dutyCyclePercentage">The duty cycle percentage to write.</param>
+        void StartWriting(int pwmChip, int pwmChannel, double frequencyInHertz, double dutyCyclePercentage);
+
+        /// <summary>
+        /// Stops writing to an open channel.
+        /// </summary>
+        /// <param name="pwmChip">The PWM chip.</param>
+        /// <param name="pwmChannel">The PWM channel.</param>
+        void StopWriting(int pwmChip, int pwmChannel);
+    }
+}

src/System.Device.Gpio/System/Device/Pwm/PwmController.cs

@@ -3,11 +3,10 @@
 // See the LICENSE file in the project root for more information.
 
 using System.Collections.Generic;
-using System.Linq;
 
 namespace System.Device.Pwm
 {
-    public sealed partial class PwmController : IDisposable
+    public sealed partial class PwmController : IPwmController
     {
         private readonly PwmDriver _driver;
         /// <summary>