This replaces the custom LED trigger code in mach-realview with
some overarching platform code for the plat-versatile family that
will lock down LEDs 2 thru 5 for CPU activity indication. The
day we have 8 core ARM systems the plat-versatile code will have
to become more elaborate.

Tested on RealView PB11MPCore by invoking four different CPU
hogs (yes > /dev/null&) and see the LEDs go on one at a time.
They all go off as the hogs are killed. Tested on the PB1176
as well - just one activity led (led 2) goes on and off with
CPU activity.

(bryan.wu@canonical.com: use ledtrig-cpu instead of ledtrig-arm-cpu)

Cc: Richard Purdie <rpurdie@rpsys.net>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Bryan Wu <bryan.wu@canonical.com>
Acked-by: Pawel Moll <pawel.moll@arm.com>