powerpc/wsp: Add PCIe Root support to PowerEN/WSP

Based on a patch by Benjamin Herrenschmidt <benh@kernel.crashing.org>

Modernized and slightly modified to not record erros into the nvram
log since we do not have that device driver just yet.

Jimi Xenidis <jimix@pobox.com>

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>