PCI: Move pci_iomap.c to drivers/pci/

The entirety of pci_iomap.c is guarded by an #ifdef CONFIG_PCI. It,
consequently, does not belong to lib/ because it is not generic
infrastructure.

Move pci_iomap.c to drivers/pci/ and implement the necessary changes to
Makefiles and Kconfigs.

Update MAINTAINERS file.

Update Documentation.

Link: https://lore.kernel.org/r/20240131090023.12331-3-pstanner@redhat.com
[bhelgaas: squash in https://lore.kernel.org/r/20240212150934.24559-1-pstanner@redhat.com]
Suggested-by: Danilo Krummrich <dakr@redhat.com>
Signed-off-by: Philipp Stanner <pstanner@redhat.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>