amd64_edac: Cleanup Dram Configuration registers handling

* Restrict DCT ganged mode check since only Fam10h supports it
* Adjust DRAM type detection for BD since it only supports DDR3
* Remove second and thus unneeded DCLR read in k8_early_channel_count() - we do
  that in read_mc_regs()
* Cleanup comments and remove family names from register macros
* Remove unused defines

There should be no functional change resulting from this patch.

Signed-off-by: Borislav Petkov <borislav.petkov@amd.com>