soc/intel: Implement PCIe RP devicetree update based on LCAP

Most of the current implementations for FSP-based platforms
make (sometimes wrong) assumptions how FSP reorders root ports
and what is specified in the devicetree. We don't have to make
assumptions though, and can read the root-port number from the
PCIe link capapilities (LCAP) instead. This is also what we do
in ASL code for years already.

This new implementation acts solely on information read from
the PCI config space. In a first round, we scan all possible
DEVFNs and store which root port has that DEVFN now. Then, we
walk through the devicetree that still only knows devices that
were originally mentioned in `devicetree.cb`, update device
paths and unlink vanished devices.

To be most compatible, we work with the following constraints:
  o Use only standard PCI config registers.
  o Most notable, don't try to read the registers that
    configure the function numbers. FSP has undocumented
    ways to block access to non-standard registers.
  o Don't make assumptions what function is assigned to
    hidden devices.

The following assumptions were made, though:
  o The absolute root-port numbering as documented in
    datasheets matches what is read from LCAP.
  o This numbering doesn't contain any gaps.
  o Original root-port function numbers below a PCI
    device start at function zero and also don't
    contain any gaps.

Change-Id: Ib17d2b6fd34608603db3936d638bdf5acb46d717
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/35985
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-by: Michael Niewöhner
Reviewed-by: Patrick Rudolph <patrick.rudolph@9elements.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

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