ENGR00275403 ARM: imx: Support CPU hotplug

ENGR00275403 ARM: imx: Support CPU hotplug

When doing secondary cores enable/disable, the enable bit
and reset bit in SRC should be written together. Without
this, CPU hotplug can NOT pass stress test, and with this,
it can passed over 500k iterations CPU hotplug test with
many threads running in background, at least three boards
are tested.

When trying hotplug a secondary core, it should stay
in idle forever before it is disabled from SRC.

Signed-off-by: Anson Huang <b20788@freescale.com>