Bug#981186: linux: Enable CMN-600 interconnect on arm64

[Noah Meyerhans <noahm@debian.org>]

On Wed, Jan 27, 2021 at 12:57:07PM +0000, Wookey wrote:
> Current arm hardware such as graviton2 (AWS arm64 hardware) has
> 'Coherent Mesh Network' interconnect (between components in a
> soc). It's important that support for this is built in the kernel so
> it can be used.
> 
> This requires CONFIG_ARM_CMN=y

To be precise, this driver is needed for perf event monitoring of this
interconnect.  The interconnect itself is always in use.

On Amazon EC2, these PMU events are only exposed on the bare-metal
instances (e.g. m6g.metal), not the VMs.

We should still enable support for this driver, in any case.

noah