[CPUFREQ] Make cpufreq suspend code conditional on powerpc.

The suspend code runs with interrupts disabled, and the powerpc workaround we
do in the cpufreq suspend hook calls the drivers ->get method.

powernow-k8's ->get does an smp_call_function_single
which needs interrupts enabled

cpufreq's suspend/resume code was added in 42d4dc3f

 to work around
a hardware problem on ppc powerbooks.  If we make all this code
conditional on powerpc, we avoid the issue above.

Signed-off-by: Dave Jones <davej@redhat.com>