pci: Honour pci_skip_dev()

When enumerating devices, honour the pci_skip_dev() function. This can
be used by PCI controller drivers to restrict which devices will be
probed.

This is required by the NVIDIA Tegra PCIe controller driver, which will
fail with a data abort exception if an access is attempted to a device
number larger than 0 outside of bus 0. pci_skip_dev() is therefore
implemented to prevent any such accesses.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Simon Glass <sjg@chromium.org>