plat: imx8m: 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.

Signed-off-by: Jacky Bai <ping.bai@nxp.com>
Change-Id: I1296592e05fa78429c3f0fac066951521db755e3
diff --git a/plat/imx/imx8m/include/gpc.h b/plat/imx/imx8m/include/gpc.h
index 6f86e1d..075da91 100644
--- a/plat/imx/imx8m/include/gpc.h
+++ b/plat/imx/imx8m/include/gpc.h
@@ -54,6 +54,8 @@
 	bool always_on;
 };
 
+DECLARE_BAKERY_LOCK(gpc_lock);
+
 /* function declare */
 void imx_gpc_init(void);
 void imx_set_cpu_secure_entry(unsigned int core_index, uintptr_t sec_entrypoint);