PCI: Update documentation 00-INDEX file

The PCI-DMA-mapping.txt moved to general docs and became DMA-API-HOWTO.txt
in 5e07c2c7

 ("Documentation: rename PCI/PCI-DMA-mapping.txt to
DMA-API-HOWTO.txt").  Add new file about PCI Express I/O Virtualization.

Signed-off-by: Erik Ekman <erik@kryo.se>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>