Add a debug config fragment that we can use to put useful debug
options into.

It can be used like:
  # make foo_defconfig
  # make debug.config

Currently the only option included is to enable debugfs SCOM access.
Suggested-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Andrew Donnellan <ajd@linux.ibm.com>
[mpe: Drop the special targets, just use the fragment directly]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20190801045855.5822-1-ajd@linux.ibm.com