PCI: brcmstb: Add link statistics debug features

Add two features that assist in diagnosing link instability issues.

The debugfs additions allow for snapshots of the Physical Layer
statistics registers to be taken, during either free-running capture or
after a hardware-controlled capture interval.

To arm the capture engine (and reset the stats counters), write an
integer N to:
/sys/kernel/debug/pcie@<addr>/stats_trigger

The engine will run forever with a value of 0, or disarm after N
microseconds.

To snapshot the hardware stats counters, write to:
/sys/kernel/debug/pcie@<addr>/stats_snapshot

Reading this file will return the snapshot. If no writes have occurred
since boot, the snapshot will be of the initial link training period.

The ltssm_trace module parameter printk's the states during initial link
startup, in situations where failure to establish the link is a fatal
error.

Signed-off-by: Jonathan Bell <jonathan@raspberrypi.com>

Author: P33M