PCI: Add PCI_EXT_CAP_ID_PL_32GT define

Add the define for PCI_EXT_CAP_ID_PL_32GT for drivers that will want this
whilst doing Gen5/Gen6 accesses.

Link: https://lore.kernel.org/r/20230531095713.293229-1-ben.dooks@codethink.co.uk


Signed-off-by: Ben Dooks <ben.dooks@sifive.com>
Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>