PCI: Rename "PCI Express support" kconfig title

The previous option title "PCI Express support" is confusing.  The name
seems to imply this option is required to get PCIe support, which is not
true.

Fix it to "PCI Express Port Bus support" which is more accurate.

Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>