kernel: tick-sched: Add API to get the next wakeup for a CPU

Add get_next_event_cpu to get the next wakeup time for the CPU. This
is used by the sleep driver if it has to query the next wakeup for a
CPU other than the thread that its running on.

Change-Id: I889de90928b9b1e51a51b2f9205d7865facfcc20
Signed-off-by: Mahesh Sivasubramanian <msivasub@codeaurora.org>