soc/intel/elkhartlake: Make PCIe root port speed limit configurable

soc/intel/elkhartlake: Make PCIe root port speed limit configurable

In cases where there are limitations on the connected device behind the
PCIe root port it can be necessary to limit the speed. The FSP parameter
'PcieRpPcieSpeed' allows to set the speed limit.

This patch provides a chip config so that this FSP parameter can be set
as needed in the devicetree on mainboard level.

Change-Id: I9fc24de1682279e4ae4c090147a6ef7995b441bc
Signed-off-by: Mario Scheithauer <mario.scheithauer@siemens.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/73766
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Lean Sheng Tan <sheng.tan@9elements.com>
Reviewed-by: Maximilian Brune <maximilian.brune@9elements.com>