feat(ec2): add m6a instances (#17764)

New `m6a` instances release notes:
https://aws.amazon.com/blogs/aws/new-amazon-ec2-m6a-instances-powered-by-3rd-gen-amd-epyc-processors/

![image](https://user-images.githubusercontent.com/31543/143961046-99e865fd-e79d-420e-8e27-02963967454b.png)


CFN docs have already been updated:
https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-properties-ec2-instance.html#cfn-ec2-instance-instancetype

----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

Author: robertd

packages/@aws-cdk/aws-ec2/lib/instance-types.ts

@@ -546,6 +546,16 @@ export enum InstanceClass {
    */
   M6I = 'm6i',
 
+  /**
+   * Standard instances based on 3rd Gen AMD EPYC processors, 6th generation.
+   */
+  STANDARD6_AMD = 'm6a',
+
+  /**
+  * Standard instances based on 3rd Gen AMD EPYC processors, 6th generation.
+  */
+  M6A = 'm6a',
+
   /**
    * Standard instances, 6th generation with Graviton2 processors and local NVME drive
    */