The SDK7786 FPGA has secondary control over the PCIe clocks, specifically
relating to the slots and oscillator. This ties the FPGA clocks in to the
clock framework and balances the refcounting similar to how the primary
on-chip clocks are managed. While the on-chip clocks are per-port, the
FPGA clock enable/disable is global for the entire block.
Signed-off-by: NPaul Mundt <lethal@linux-sh.org>

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: NPaul Mundt <lethal@linux-sh.org>