EDAC, ghes: Map dimms to groups during ghes_edac_dmidecode()

Now that we have a structure that decribes a group of dimms, we want
to add the dimms to it. This is done during the second walk of the
dmitable.

We find the corresponding group based on this dimm's phys_mem_array_handle,
then add the dimm's handle to it.

This will eventually let us find group and dimm indexes based
only on the dimm handle (when its provided with an error report).

Signed-off-by: James Morse <james.morse@arm.com>