cpufreq: SPEAr: Make read-only array sys_clk_src static

Don't populate the read-only array sys_clk_src on the stack but instead
make it static and add in a missing const. Also makes the object code a
little smaller.

Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>