arch: Create a base class for decoders.

arch: Create a base class for decoders.

This base class doesn't actually hold anything yet, it's just a place to
add shared functionality or interfaces later.

Change-Id: Ia33217bd78b1d1ff3df3b2202095576a4e5d8153
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/32897
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>