sched: Move data structures under CONFIG_SCHED_HMP

Frequency-demand conversion data structures are only used under
CONFIG_SCHED_HMP. Move them out of sched.h into hmp.c to where they
actually belong after the recent refactor.

Change-Id: I3c3eebca86062f11b80af93ba3716695eb787376
Signed-off-by: Syed Rameez Mustafa <rameezmustafa@codeaurora.org>