ARM: realview: delete the RealView board files

The device tree boot is now feature-complete and fully working,
so remove all the RealView board files. Move the remaining
realview_cpu_die() function frome "core.h" to the new "hotplug.h"
and leave just a minimal device tree core.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>