Documentation: PCI: Give unique labels to sections

Make subsection labels more specific to avoid sphinx warnings.

Exact warning:
 Documentation/PCI/endpoint/pci-endpoint.rst:208: WARNING: duplicate label
pci/endpoint/pci-endpoint:other apis, other instance in Documentation/PCI/endpoint/pci-endpoint.rst

Link: https://lore.kernel.org/r/20200503214926.23748-1-bryce.steven.willey@gmail.com


Signed-off-by: Bryce Willey <Bryce.Steven.Willey@gmail.com>
[lorenzo.pieralisi@arm.com: commit log]
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Acked-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Rob Herring <robh@kernel.org>