sched: hmp: Optimize cycle counter reads

The cycle counter read is a bit of an expensive operation and requires
locking across all CPUs in a cluster. Optimize this by returning the
same value if the delta between two reads is zero (so if two reads are
done in the same sched context) or if the last read was within a
specific time period prior to the current read.

Change-Id: I99da5a704d3652f53c8564ba7532783d3288f227
Signed-off-by: Vikram Mulukutla <markivx@codeaurora.org>