ARM: imx: do not return from imx_cpu_die() call

When imx_cpu_die() is being called, the cpu should never return from the
call but just in WFI and wait for hardware to take it down.  So let's
do cpu_do_idle() repeatly in the call.  Doing this help improve the
relibility of hotplug operation.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>