[POWERPC] PMU LED whitelisting of PowerMac 7,2 and 7,3

This allows the PMU LED on both a PowerMac 7,2 (Dual G5 2.0GHz, June 2003)
and a PowerMac 7,3 (Dual G5 2.0GHz, June 2004) to be controlled.
The physical LED is never off, unlike an iBook/PowerBook LED.
It is rather dim ("off") or very bright ("on").

Signed-off-by: Tony Vroon <chainsaw@gentoo.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>