ARM: Add ARCH_HAS_CPUFREQ for presence of CPUFREQ driver

Add ARCH_HAS_CPUFREQ so that each machine config can select
it if they have CPUFREQ driver support. This means that the
CPUFREQ specific area does not need the if statement updating
each time a new machine is added.

Signed-off-by: Ben Dooks <ben@simtec.co.uk>
Signed-off-by: Ben Dooks <ben-linux@fluff.org>