refactor(imx8mq): introduce BL31_SIZE

No functional change.

Introduce BL31_SIZE define and calculate the limits based on the
BL31_BASE and the BL31_SIZE define. Also make use of SZ_64K to make
it easier to read. This is required for later BL31 PIE support since
it drops the calculation based on the BL31_LIMIT and BL31_BASE.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Change-Id: I517074b866b5bf11841b51777f87c926b304488d
diff --git a/plat/imx/imx8m/imx8mq/imx8mq_bl31_setup.c b/plat/imx/imx8m/imx8mq/imx8mq_bl31_setup.c
index 195db02..61c92a2 100644
--- a/plat/imx/imx8m/imx8mq/imx8mq_bl31_setup.c
+++ b/plat/imx/imx8m/imx8mq/imx8mq_bl31_setup.c
@@ -176,7 +176,7 @@
 void bl31_plat_arch_setup(void)
 {
 	const mmap_region_t bl_regions[] = {
-		MAP_REGION_FLAT(BL31_BASE, BL31_LIMIT - BL31_BASE,
+		MAP_REGION_FLAT(BL31_BASE, BL31_SIZE,
 				MT_MEMORY | MT_RW | MT_SECURE),
 		MAP_REGION_FLAT(BL_CODE_BASE, BL_CODE_END - BL_CODE_BASE,
 				MT_MEMORY | MT_RO | MT_SECURE),