EDAC, skx, i10nm: Make skx_common.c a pure library

EDAC, skx, i10nm: Make skx_common.c a pure library

The following Kconfig constellations fail randconfig builds:

  CONFIG_ACPI_NFIT=y
  CONFIG_EDAC_DEBUG=y
  CONFIG_EDAC_SKX=m
  CONFIG_EDAC_I10NM=y

or

  CONFIG_ACPI_NFIT=y
  CONFIG_EDAC_DEBUG=y
  CONFIG_EDAC_SKX=y
  CONFIG_EDAC_I10NM=m

with:
  ...
  CC [M]  drivers/edac/skx_common.o
  ...
  .../skx_common.o:.../skx_common.c:672: undefined reference to `__this_module'

That is because if one of the two drivers - skx_edac or i10nm_edac - is
built-in and the other one is a module, the shared file skx_common.c
gets linked into a module object by kbuild. Therefore, when linking that
same file into vmlinux, the '__this_module' symbol used in debugfs isn't
defined, leading to the above error.

Fix it by moving all debugfs code from skx_common.c to both skx_base.c
and i10nm_base.c respectively. Thus, skx_common.c doesn't refer to the
'__this_module' symbol anymore.

Clarify skx_common.c's purpose at the top of the file for future
reference, while at it.

 [ bp: Make text more readable. ]

Fixes: d4dc89d069aa ("EDAC, i10nm: Add a driver for Intel 10nm server processors")
Reported-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Qiuxu Zhuo <qiuxu.zhuo@intel.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: James Morse <james.morse@arm.com>
Cc: Mauro Carvalho Chehab <mchehab@kernel.org>
Cc: linux-edac <linux-edac@vger.kernel.org>
Link: https://lkml.kernel.org/r/20190321221339.GA32323@agluck-desk