PCI: pciehp: add auto option to pciehp_detect_mode

ACPI based hot-pluggable PCIe slot detection logic was added to
prevent the problem non hot-pluggable PCIe slot was detected as
hot-pluggable. The slot detection logic can be selected through
'pciehp_detect_mode', but it would be better if it is selected
automatically.

This patch adds 'auto' option for 'pciehp_detect_mode'. When it is
specified, pciehp judges which 'acpi' or 'pcie' should be used. It
seems that the physical slot number is duplicated among some slots on
most of the platforms with the above-mentioned problem. So 'auto' mode
uses this information to judge which 'acpi' or 'pcie' should be
used. That is, if duplicated physical slot numbers are detected,
'acpi' mode is used. This method is not perfect, but it's realistic.

Signed-off-by: Kenji Kaneshige <kaneshige.kenji@jp.fujitsu.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>