PCI/P2PDMA: Add missing struct p2pdma_provider documentation

PCI/P2PDMA: Add missing struct p2pdma_provider documentation

Two fields in struct p2pdma_provider were not documented, which resulted
in the following kernel-doc warning:

  Warning: include/linux/pci-p2pdma.h:26 struct member 'owner' not described in 'p2pdma_provider'
  Warning: include/linux/pci-p2pdma.h:26 struct member 'bus_offset' not described in 'p2pdma_provider'

Repro:

  $ scripts/kernel-doc -none include/linux/pci-p2pdma.h

Fixes: f58ef9d1d135 ("PCI/P2PDMA: Separate the mmap() support from the core logic")
Reported-by: Bjorn Helgaas <helgaas@kernel.org>
Closes: https://lore.kernel.org/all/20260102234033.GA246107@bhelgaas
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Logan Gunthorpe <logang@deltatee.com>
Link: https://patch.msgid.link/20260104-fix-p2p-kdoc-v1-1-6d181233f8bc@nvidia.com