cpufreq: pxa: make pxa_freqs arrays const

pxa255_run_freqs and pxa255_turbo_freqs are only read.
This patch updates arrays declaration, find_freq_tables()
and its callsites.

Suggested-by: Joe Perches <joe@perches.com>
Signed-off-by: Fabian Frederick <fabf@skynet.be>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>