sched: Add documentation for the revised hmp zone scheduler.

Add documentation for the revised task placement logic for the
scheduler. Since the old file sched-hmp.txt is still required,
add a new one instead.

Change-Id: Ic7e3845c8d6b85b7918cd35c2a0a482a621fe525
Signed-off-by: Syed Rameez Mustafa <rameezmustafa@codeaurora.org>