refactor(imx8mm): introduce BL2_SIZE and BL31_SIZE

No functional change.

Introduce BLx_SIZE defines and calculate the limits based on the
BLx_BASE and the BLx_SIZE define. Also make use of SZ_128K 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: Marco Felsch <m.felsch@pengutronix.de>
Change-Id: I8670faa5d5a572ef230011594f3d0d594fb257d9
diff --git a/plat/imx/imx8m/imx8mm/imx8mm_bl31_setup.c b/plat/imx/imx8m/imx8mm/imx8mm_bl31_setup.c
index 13a4fa6..d63e0a5 100644
--- a/plat/imx/imx8m/imx8mm/imx8mm_bl31_setup.c
+++ b/plat/imx/imx8m/imx8mm/imx8mm_bl31_setup.c
@@ -175,7 +175,7 @@
 }
 
 #define MAP_BL31_TOTAL										   \
-	MAP_REGION_FLAT(BL31_BASE, BL31_LIMIT - BL31_BASE, MT_MEMORY | MT_RW | MT_SECURE)
+	MAP_REGION_FLAT(BL31_BASE, 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									   \