armv8: Move secure_ram variable out of generic global data

Secure_ram variable was put in generic global data. But only ARMv8
uses this variable. Move it to ARM specific data structure.

Signed-off-by: York Sun <york.sun@nxp.com>
diff --git a/common/board_f.c b/common/board_f.c
index d405b5b..0fc96bd 100644
--- a/common/board_f.c
+++ b/common/board_f.c
@@ -339,7 +339,7 @@
 	 * Record secure memory location. Need recalcuate if memory splits
 	 * into banks, or the ram base is not zero.
 	 */
-	gd->secure_ram = gd->ram_size;
+	gd->arch.secure_ram = gd->ram_size;
 #endif
 	/*
 	 * Subtract specified amount of memory to hide so that it won't