thunderbolt: Add debugfs interface

thunderbolt: Add debugfs interface

This adds debugfs interface that can be used for debugging possible
issues in hardware/software. It exposes router and adapter config spaces
through files like this:

  /sys/kernel/debug/thunderbolt/<DEVICE>/regs
  /sys/kernel/debug/thunderbolt/<DEVICE>/<PORT1>/regs
  /sys/kernel/debug/thunderbolt/<DEVICE>/<PORT1>/path
  /sys/kernel/debug/thunderbolt/<DEVICE>/<PORT1>/counters
  /sys/kernel/debug/thunderbolt/<DEVICE>/<PORT2>/regs
  /sys/kernel/debug/thunderbolt/<DEVICE>/<PORT2>/path
  /sys/kernel/debug/thunderbolt/<DEVICE>/<PORT2>/counters
  ...

The "regs" is either the router or port configuration space register
dump. The "path" is the port path configuration space and "counters" is
the optional counters configuration space.

These files contains one register per line so it should be easy to use
normal filtering tools to find the registers of interest if needed.

The router and adapter regs file becomes writable when
CONFIG_USB4_DEBUGFS_WRITE is enabled (which is not supposed to be done
in production systems) and in this case the developer can write "offset
value" lines there to modify the hardware directly. For convenience this
also supports the long format the read side produces (but ignores the
additional fields). The counters file can be written even when
CONFIG_USB4_DEBUGFS_WRITE is not enabled and it is only used to clear
the counter values.

Signed-off-by: Gil Fine <gil.fine@intel.com>
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>