Apply MLK-22578 from imx8mn to imx8mq code

MLK-22578 plat: imx8mn: Fix the race condition during cpu hotplug
CPU hotplug & cpuidle have some race condition when doing CPU hotplug
stress test. different CPU cores have the chance to access the same
GPC register(A53_AD), so lock is necessary to do exlusive access.

Change-Id: I3ff5cbe6bf333c9555591a313c1a41dbdb688cee
Signed-off-by: Leonid Lobachev <leonidl@google.com>