[POWERPC] Add cpufreq support for Xserve G5

The Xserve G5 are capable of frequency switching like other desktop G5s.
This enables it. It also fix a Kconfig issue which prevented from
building the G5 cpufreq support if CONFIG_PMAC_SMU was not set (the
first version of that driver only worked with SMU based macs, but this
isn't the case anymore).

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