Documentation: PCI: convert pci.txt to reST

Convert plain text documentation to reStructuredText format and add it to
Sphinx TOC tree.  No essential content change.

Move the description of struct pci_driver and struct pci_device_id into
in-source comments.

Signed-off-by: Changbin Du <changbin.du@gmail.com>
[bhelgaas: fix kernel-doc warnings related to moving descriptions to
linux/pci.h, fix "space tab" whitespace errors in mod_devicetable.h]
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>