PCI: Update pcie_ports 'auto' behavior for non-ACPI platforms

The pcie_ports parameter, which defaults to 'auto', allows a user
to specify if PCIe port services are disabled ('compat'), always
enabled ('native'), or only used when allowed by the BIOS
('auto').

Where CONFIG_ACPI isn't enabled, as is often the case for non
x86/ia64 platforms, the 'auto' behavior results in that of
'compat'. Thus in order to use port services on these platforms
'pcie_ports=native' must be added to the kernel command line.

This patch results in the 'native' behavior being followed where
'auto' is selected and ACPI is not enabled.

Signed-off-by: Andrew Murray <amurray@embedded-bits.co.uk>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>