helpers: backend_device: Add Node accessor

Allow the application to access the device node class through the
BackendDevice object.

Signed-off-by: Naushir Patuck <naush@raspberrypi.com>

Author: naushir

src/helpers/backend_device.hpp

@@ -30,6 +30,11 @@ class BackendDevice
 		return valid_;
 	}
 
+	V4l2Device &Node(const std::string &node)
+	{
+		return nodes_.at(node);
+	}
+
 	std::map<std::string, V4l2Device::Buffer> AcquireBuffers();
 	void ReleaseBuffer(const std::map<std::string, V4l2Device::Buffer> &buffers);
 	V4l2Device::Buffer &ConfigBuffer()