parisc: led: Rewrite LED/LCD driver to utilizize Linux LED subsystem

Rewrite the whole driver and drop the own code to calculate load
average, disk and LAN load. Switch instead to use the in-kernel LED
subsystem, which gives us quite some advantages, e.g.
- existing triggers for heartbeat and disk/lan activity can be used
- users can configre the LEDs at will to any existing trigger via
  /sys/class/leds
- less overhead since we don't need to run own timers
- fully integrated in Linux and as such cleaner code.

Note that the driver now depends on CONFIG_LEDS_CLASS which has to
be built-in and not as module.

Signed-off-by: Helge Deller <deller@gmx.de>