In preparation for future changes to the scheduler let's add some
tracing that makes it easy for us to see what's happening.  By default
this tracing will be off.

By changing "core.h" you can easily trace to ftrace, the console, or
nowhere.
Acked-by: John Youn <johnyoun@synopsys.com>
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>
Tested-by: Heiko Stuebner <heiko@sntech.de>
Tested-by: Stefan Wahren <stefan.wahren@i2se.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>