sched: Add API to set task's initial task load

Add a per-task attribute, init_load_pct, that is used to initialize
newly created children's initial task load. This helps important
applications launch their child tasks on cpus with highest capacity.

Change-Id: Ie9665fd2aeb15203f95fd7f211c50bebbaa18727
Signed-off-by: Srivatsa Vaddagiri <vatsa@codeaurora.org>