mb/siemens/mc_ehl2: Adjust PCIe clock settings in devicetree

mb/siemens/mc_ehl2: Adjust PCIe clock settings in devicetree

On mc_ehl2 there are currently four of the six PCIe clocks used to drive
PCIe devices. None of the used clock output is dedicated to a special
device. Therefore do not use a port mapping of the clocks to avoid a
stopping clock once a device is missing and the matching root port is
disabled. Instead set the mapping to 'PCIE_CLK_FREE' to have a free
running clock.

In addition, use the defined constant 'PCIE_CLK_NOTUSED' instead of the
value 0xFF to disable the CLKREQ-feature and unused clocks.

Change-Id: I81419887b7f463a937917b971465245c1cb46b94
Signed-off-by: Mario Scheithauer <mario.scheithauer@siemens.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/58852
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Werner Zeh <werner.zeh@siemens.com>
Reviewed-by: Lean Sheng Tan <lean.sheng.tan@intel.com>