powerpc/pseries: Define PCI bus speed for Gen4 and Gen5

Update bus speed definition for PCI Gen4 and 5.

Signed-off-by: Frederic Barrat <fbarrat@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20201130152949.26467-1-fbarrat@linux.ibm.com