base: Add callback system

This patch introduces new API that will allow workloads to call host callback
functions. The callback system is hooked into the scheduler and will schedule
out workloads from the MPUs while callbacks are being processed on the host.

Signed-off-by: Yuji Mano <yuji.mano@am.sony.com>
Acked-by: Kazunori Asayama <asayama@sm.sony.co.jp>