sched: Fix possible overflow in cpu_cycles_to_freq()

Truncating period to u32 could lead to incorrect results. Make it u64
instead.

Change-Id: I5224a943e64bc6d64b6c8e614a01f798a6cdc796
Signed-off-by: Puja Gupta <pujag@codeaurora.org>
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>