parisc: led: Reduce CPU overhead for disk & lan LED computation

parisc: led: Reduce CPU overhead for disk & lan LED computation

Older PA-RISC machines have LEDs which show the disk- and LAN-activity.
The computation is done in software and takes quite some time, e.g. on a
J6500 this may take up to 60% time of one CPU if the machine is loaded
via network traffic.

Since most people don't care about the LEDs, start with LEDs disabled and
just show a CPU heartbeat LED. The disk and LAN LEDs can be turned on
manually via /proc/pdc/led.

Signed-off-by: Helge Deller <deller@gmx.de>
Cc: <stable@vger.kernel.org>