sched/walt: factor out WALT from HMP scheduler

At present, hmp.c contains HMP task placement logic as well as WALT.
Factor out WALT logic into a separate file walt.c so EAS can use WALT as
CPU load tracking and frequency guidance algorithm.

Also set sched_io_is_busy = 1 by default.

Change-Id: I96c3fd7d637b9bfa283083a1a6ff6d5dd261e0fb
Signed-off-by: Joonwoo Park <joonwoop@codeaurora.org>