PCI: Add pci_get_base_class() helper

There is no function to get all PCI devices in a system by matching
against the base class code only, ignoring the sub-class code and
the programming interface.  Add pci_get_base_class() to suit the
need.

For example, if a driver wants to process all PCI display devices in
a system, it can do so like this:

  pdev = NULL;
  while ((pdev = pci_get_base_class(PCI_BASE_CLASS_DISPLAY, pdev))) {
    do_something_for_pci_display_device(pdev);
  }

Link: https://lore.kernel.org/r/20230825062714.6325-2-sui.jingfeng@linux.dev
Signed-off-by: Sui Jingfeng <suijingfeng@loongson.cn>
[bhelgaas: reword commit log]
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>