powerpc/powernv: Remove support for p5ioc2

"p5ioc2 is used by approximately 2 machines in the world, and has never
ever been a supported configuration."

The code for p5ioc2 is essentially unused and complicates what is already
a very complicated codebase.  Its removal is essentially a "free win" in
the effort to simplify the powernv PCI code.

In addition, support for p5ioc2 has been dropped from skiboot.  There's no
reason to keep it around in the kernel.

Signed-off-by: Russell Currey <ruscur@russell.cc>
Acked-by: Gavin Shan <gwshan@linux.vnet.ibm.com>
Acked-by: Stewart Smith <stewart@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>