Only try to enable a 64-bit window on AMD CPUs when "pci=big_root_window"
is specified.

This taints the kernel because the new 64-bit window uses address space we
don't know anything about, and it may contain unreported devices or memory
that would conflict with the window.

The pci_amd_enable_64bit_bar() quirk that enables the window is specific to
AMD CPUs.  The generic solution would be to have the firmware enable the
window and describe it in the host bridge's _CRS method, or at least
describe it in the _PRS method so the OS would have the option of enabling
it.

Signed-off-by: Christian König <christian.koenig@amd.com>
[bhelgaas: changelog, extend doc, mention taint in dmesg]
Signed-off-by: Bjorn Helgaas <helgaas@kernel.org>