sched: Further re-factor HMP specific code

The structures being moved around are only used for trace events
defined under CONFIG_SCHED_HMP. Move code to hmp.c to reflect
the same.

Change-Id: Ib959355264405ab779b24948f111a2ca61d367de
Signed-off-by: Syed Rameez Mustafa <rameezmustafa@codeaurora.org>