kernel: tickless: Add function to check if list contains multiple nodes

Scheduler needs to do time slicing only if there are multiple threads
active with the same priority. This function checks if the list has
more than one node. This would be used to check the list containing
threads with same priority for multiple nodes.

Jira: ZEP-339
Change-Id: I8c7daf77a6540c642ce58a3763b26cd1e06ddc30
Signed-off-by: Ramesh Thomas <ramesh.thomas@intel.com>