Add MCR support

I should have a merge request implementing DSS and EU fuse reading (#114 - closed) ready soon. After that's in place, we'll be ready to start implementing proper handling of Multicast/Replicated (MCR) registers in the Xe driver. This i915 file's kerneldoc explains what MCR registers are and how we need to handle them. We expect the general interfaces to operate on MCR registers should be very similar to the interfaces we use on i915.