FVP: Do not map DEVICE2 memory range when TBB is disabled

The DEVICE2 memory range is needed to access the Root of Trust Public
Key registers. This is not needed when Trusted Board Boot is disabled
so it's safer to not map it in this case. This also saves one level-2
page table in each of BL1 and BL2 images.

Also add some comments.

Change-Id: I67456b44f3fd5e145f6510a8499b7fdf720a7273
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>

plat/arm/board/common/board_css_common.c

@@ -19,6 +19,7 @@ const mmap_region_t plat_arm_mmap[] = {
 	CSS_MAP_DEVICE,
 	SOC_CSS_MAP_DEVICE,
 #if TRUSTED_BOARD_BOOT
+	/* Map DRAM to authenticate NS_BL2U image. */
 	ARM_MAP_NS_DRAM1,
 #endif
 	{0}

plat/arm/board/fvp/fvp_common.c

@@ -36,6 +36,10 @@ arm_config_t arm_config;
 					DEVICE1_SIZE,			\
 					MT_DEVICE | MT_RW | MT_SECURE)
 
+/*
+ * Need to be mapped with write permissions in order to set a new non-volatile
+ * counter value.
+ */
 #define MAP_DEVICE2	MAP_REGION_FLAT(DEVICE2_BASE,			\
 					DEVICE2_SIZE,			\
 					MT_DEVICE | MT_RW | MT_SECURE)
@@ -56,8 +60,10 @@ const mmap_region_t plat_arm_mmap[] = {
 	V2M_MAP_IOFPGA,
 	MAP_DEVICE0,
 	MAP_DEVICE1,
-	MAP_DEVICE2,
 #if TRUSTED_BOARD_BOOT
+	/* To access the Root of Trust Public Key registers. */
+	MAP_DEVICE2,
+	/* Map DRAM to authenticate NS_BL2U image. */
 	ARM_MAP_NS_DRAM1,
 #endif
 	{0}
@@ -70,9 +76,12 @@ const mmap_region_t plat_arm_mmap[] = {
 	V2M_MAP_IOFPGA,
 	MAP_DEVICE0,
 	MAP_DEVICE1,
-	MAP_DEVICE2,
 	ARM_MAP_NS_DRAM1,
 	ARM_MAP_TSP_SEC_MEM,
+#if TRUSTED_BOARD_BOOT
+	/* To access the Root of Trust Public Key registers. */
+	MAP_DEVICE2,
+#endif
 #if ARM_BL31_IN_DRAM
 	ARM_MAP_BL31_SEC_DRAM,
 #endif