sh: pci: Support slot 4 routing on SDK7786.

SDK7786 supports connecting either slot3 or 4 to the same PCIe port by
way of FPGA muxing. By default the vertical slot 3 on the baseboard is
enabled, so this adds in a command line option for forcibly enabling the
slot 4 edge connector.

If nothing has been specified on the command line, we fall back to
reading the resistor values for card presence to figure out where to
route the port to.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>