Remove platform_cpu_idle() and platform_cpu_idle_wakeup() interfaces

These interfaces were put in place to let QorIQ SoCs dictate CPU idling
semantics, in order to support capabilities such as NAP mode and deep sleep.
However, this never stabilized, and the idling support reverted back to
CPU-level rather than SoC level. Move this code back to cpu.c instead. If
at a later date the lower power modes do come to fruition, it should be done
by overriding the cpu_idle_hook instead of this platform hook.