feat(plat/mediatek/mt8186): add CPU hotplug

Implement PSCI platform operations to support CPU hotplug and MCDI.

TEST=bringup 8 CPUs successfully on kernel stage.
BUG=b:202871018

Change-Id: Ibd5423b70b3ca3f91edaa48d7ca5bc094e751510
diff --git a/plat/mediatek/mt8186/include/platform_def.h b/plat/mediatek/mt8186/include/platform_def.h
index 5a56556..479a8d4 100644
--- a/plat/mediatek/mt8186/include/platform_def.h
+++ b/plat/mediatek/mt8186/include/platform_def.h
@@ -19,6 +19,8 @@
 #define MTK_DEV_RNG2_BASE	MT_GIC_BASE
 #define MTK_DEV_RNG2_SIZE	0x600000
 
+#define SPM_BASE		(IO_PHYS + 0x00006000)
+
 /*******************************************************************************
  * GPIO related constants
  ******************************************************************************/