Add support for BL2 in XIP memory

In some use-cases BL2 will be stored in eXecute In Place (XIP) memory,
like BL1. In these use-cases, it is necessary to initialize the RW sections
in RAM, while leaving the RO sections in place. This patch enable this
use-case with a new build option, BL2_IN_XIP_MEM. For now, this option
is only supported when BL2_AT_EL3 is 1.

Signed-off-by: Jiafei Pan <Jiafei.Pan@nxp.com>
diff --git a/make_helpers/defaults.mk b/make_helpers/defaults.mk
index 751f834..77eb157 100644
--- a/make_helpers/defaults.mk
+++ b/make_helpers/defaults.mk
@@ -30,6 +30,10 @@
 # Execute BL2 at EL3
 BL2_AT_EL3			:= 0
 
+# BL2 image is stored in XIP memory, for now, this option is only supported
+# when BL2_AT_EL3 is 1.
+BL2_IN_XIP_MEM			:= 0
+
 # By default, consider that the platform may release several CPUs out of reset.
 # The platform Makefile is free to override this value.
 COLD_BOOT_SINGLE_CPU		:= 0