feat(imx8mn): add BL31 PIE support

Enable PIE support so the BL31 firmware can be loaded from anywhere
within the OCRAM (SRAM). For the PIE support we only need to replace the
BL31_BASE define by the BL31_START symbol which is a relocatable and we
need to enable it by setting ENABLE_PIE := 1.

Signed-off-by: Marco Felsch <m.felsch@pengutronix.de>
Change-Id: I33c8e35c35112d70d2725eebe484a853a8aad9e0

plat/imx/imx8m/imx8mn/imx8mn_bl31_setup.c

@@ -180,7 +180,7 @@ void bl31_early_platform_setup2(u_register_t arg0, u_register_t arg1,
 }
 
 #define MAP_BL31_TOTAL										   \
-	MAP_REGION_FLAT(BL31_BASE, BL31_SIZE, MT_MEMORY | MT_RW | MT_SECURE)
+	MAP_REGION_FLAT(BL31_START, BL31_SIZE, MT_MEMORY | MT_RW | MT_SECURE)
 #define MAP_BL31_RO										   \
 	MAP_REGION_FLAT(BL_CODE_BASE, BL_CODE_END - BL_CODE_BASE, MT_MEMORY | MT_RO | MT_SECURE)
 #define MAP_COHERENT_MEM									   \

plat/imx/imx8m/imx8mn/platform.mk

@@ -47,6 +47,7 @@ BL31_SOURCES		+=	plat/imx/common/imx8_helpers.S			\
 				${IMX_GIC_SOURCES}				\
 				${XLAT_TABLES_LIB_SRCS}
 
+ENABLE_PIE		:=	1
 USE_COHERENT_MEM	:=	1
 RESET_TO_BL31		:=	1
 A53_DISABLE_NON_TEMPORAL_HINT := 0