[PATCH] powerpc: Fix clock spreading setting on some powermacs

The code that sets the clock spreading feature of the Intrepid ASIC
must not be run on some machine models or those won't boot. This
fixes it.

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