PCI/P2PDMA: Introduce configfs/sysfs enable attribute helpers

Users of the P2PDMA infrastructure will typically need a way for the user
to tell the kernel to use P2P resources.  Typically this will be a simple
on/off boolean operation but sometimes it may be desirable for the user to
specify the exact device to use for the P2P operation.

Add new helpers for attributes which take a boolean or a PCI device.  Any
boolean as accepted by strtobool() turn P2P on or off (such as 'y', 'n',
'1', '0', etc).  Specifying a full PCI device name/BDF will select the
specific device.

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