fix(mt8188): add mmap entry for CPU idle SRAM

CPU PM driver accesses CPU idle SRAM during the system suspend
process. The region of CPU idle SRAM needs to be added as mmap entry.
Otherwise, the execption would occur.

BUG=b:244215539
TEST=Test of suspend resume passes.

Change-Id: I5838964fd9cb1b833e4006e2123febb4a4601003
diff --git a/plat/mediatek/mt8188/include/platform_def.h b/plat/mediatek/mt8188/include/platform_def.h
index 7a30357..156a7e2 100644
--- a/plat/mediatek/mt8188/include/platform_def.h
+++ b/plat/mediatek/mt8188/include/platform_def.h
@@ -178,5 +178,6 @@
 #define PLAT_CPU_PM_B_BUCK_ISO_ID	(6)
 #define PLAT_CPU_PM_ILDO_ID		(6)
 #define CPU_IDLE_SRAM_BASE		(0x11B000)
+#define CPU_IDLE_SRAM_SIZE		(0x1000)
 
 #endif /* PLATFORM_DEF_H */