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.

(bryan.wu@canonical.com:
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 <rpurdie@rpsys.net>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Bryan Wu <bryan.wu@canonical.com>
Reviewed-by: Jamie Iles <jamie@jamieiles.com>
Tested-by: Jochen Friedrich <jochen@scram.de>
ledtrig-cpu: kill useless mutex to fix sleep in atomic context

Seeing the following every time the CPU enters or leaves idle on a
Beagleboard:

BUG: sleeping function called from invalid context at kernel/mutex.c:269
in_atomic(): 1, irqs_disabled(): 0, pid: 0, name: swapper/0
no locks held by swapper/0/0.
[<c001659c>] (unwind_backtrace+0x0/0xf8) from [<c05aaa7c>] (mutex_lock_nested+0x24/0x380)
[<c05aaa7c>] (mutex_lock_nested+0x24/0x380) from [<c043bd1c>] (ledtrig_cpu+0x38/0x88)
[<c043bd1c>] (ledtrig_cpu+0x38/0x88) from [<c000f4b0>] (cpu_idle+0xf4/0x120)
[<c000f4b0>] (cpu_idle+0xf4/0x120) from [<c07e47c8>] (start_kernel+0x2bc/0x30c)

Miles Lane has reported seeing similar splats during system suspend.

The mutex in struct led_trigger_cpu appears to have no function: it
resides in a per-cpu data structure which never changes after the
trigger is registered.  So just remove it.

Reported-by: Miles Lane <miles.lane@gmail.com>
Signed-off-by: Nathan Lynch <ntl@pobox.com>
Signed-off-by: Bryan Wu <roc@roc-samos.(none)>
Integrate ledtrig-cpu.

Backport of fa8bbb13 from Linus master.

Signed-off-by: Michal Suchanek <hramrach@gmail.com>