refactor(plat/allwinner): allow custom BL31 offset

Not all Allwinner SoCs have the same arrangement to SRAM A2.

Allow to specify a offset at which BL31 will stay in SRAM A2.

Change-Id: I574140ffd704a796fae0a5c2d0976e85c7fcbdf9
Signed-off-by: Icenowy Zheng <icenowy@sipeed.com>
diff --git a/plat/allwinner/common/include/platform_def.h b/plat/allwinner/common/include/platform_def.h
index 56a2ad6..49951e0 100644
--- a/plat/allwinner/common/include/platform_def.h
+++ b/plat/allwinner/common/include/platform_def.h
@@ -25,7 +25,8 @@
 
 #else	/* !SUNXI_BL31_IN_DRAM */
 
-#define BL31_BASE			(SUNXI_SRAM_A2_BASE + 0x4000)
+#define BL31_BASE			(SUNXI_SRAM_A2_BASE + \
+					 SUNXI_SRAM_A2_BL31_OFFSET)
 #define BL31_LIMIT			(SUNXI_SRAM_A2_BASE + \
 					 SUNXI_SRAM_A2_SIZE - SUNXI_SCP_SIZE)