led-triggers: create a trigger for CPU activity

led-triggers: create a trigger for CPU activity

Attempting to consolidate the ARM LED code, this removes the
custom RealView LED trigger code to turn LEDs on and off in
response to CPU activity and replace it with a standard trigger.

([email protected]:
It introduces several syscore stubs into this trigger.
It also provides ledtrig_cpu trigger event stub in <linux/leds.h>.
Although it was inspired by ARM work, it can be used in other arch.)

Cc: Richard Purdie <[email protected]>
Signed-off-by: Linus Walleij <[email protected]>
Signed-off-by: Bryan Wu <[email protected]>
Reviewed-by: Jamie Iles <[email protected]>
Tested-by: Jochen Friedrich <[email protected]>