PCI/P2PDMA: Add sysfs group to display p2pmem stats

Add a sysfs group to display statistics about P2P memory that is registered
in each PCI device.

Attributes in the group display the total amount of P2P memory, the amount
available and whether it is published or not.

Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>