sched: walt: Add an API for querying default related thread group load

The default related thread group which is tied to schedtune is
always active. Add an API to return this group load in percentage
scaled to the minimum capacity CPU in the system.

Change-Id: Iea20137c7c2963583da249cbf97d7098d1659b66
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>