powerpc/cell/cpufreq: Add spu aware cpufreq governor

This patch adds a cpufreq governor that takes the number of running spus
into account. It's very similar to the ondemand governor, but not as complex.
Instead of hacking spu load into the ondemand governor it might be easier to
have cpufreq accepting multiple governors per cpu in future.
Don't know if this is the right way, but it would keep the governors simple.

Signed-off-by: Christian Krafft <krafft@de.ibm.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Dave Jones <davej@redhat.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>