ARM: substitute arch_idle()

Now that all implementations of arch_idle() are equivalent to cpu_do_idle()
we can just use the later directly and stop including mach/system.h.

Signed-off-by: Nicolas Pitre <nicolas.pitre@linaro.org>
Acked-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Acked-and-tested-by: Jamie Iles <jamie@jamieiles.com>
Acked-by: Tony Lindgren <tony@atomide.com>
Tested-by: Stephen Warren <swarren@nvidia.com>