[POWERPC] Xserve cpu-meter driver

This is a small driver for the Xserve G5 CPU-meter blue LEDs on the
front-panel. It might work on the Xserve G4 as well though that was
not tested. It's pretty basic and could use some improvements if
somebody cares doing them. :)

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>