Improve DCN Device Support (#8878)

DO NOT DELETE THIS TEXT

#### Please note

> Please read this information carefully. You can run `./scripts/pre-commit.php` to check your code before submitting.

- [X] Have you followed our [code guidelines?](http://docs.librenms.org/Developing/Code-Guidelines/)

#### Testers

If you would like to test this pull request then please run: `./scripts/github-apply <pr_id>`, i.e `./scripts/github-apply 5926`
fix #8755 

I have not added test data as it would require filterting quite some LLDP/IP's etc information.

includes/definitions/dcn-software.yaml

@@ -2,6 +2,8 @@ os: dcn-software
 text: 'Digital China Networks'
 type: network
 icon: dcn
+mib_dir:
+    - dcn
 over:
     - { graph: device_bits, text: 'Device Traffic' }
     - { graph: device_processor, text: 'Processor Usage' }

includes/definitions/discovery/dcn-software.yaml

@@ -0,0 +1,8 @@
+mib: DCN-MIB
+modules:
+    processors:
+          data:
+              -
+                  oid: switchCpuUsage
+                  num_oid: '.1.3.6.1.4.1.6339.100.1.11.10.{{ $index }}'
+                  descr: CPU Usage