PCI: Document that most pci options are shared between i386 and x86-64

Since the code is shared pretty much most of the pci= options are shared,
but kernel-parameters.txt marked most of them as i386 only.

Signed-off-by: Andi Kleen <ak@linux.intel.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>