build: restrict usage of CTX_INCLUDE_EL2_REGS

CTX_INCLUDE_EL2_REGS is used to save/restore EL2 registers and
it should be only used when there is SPMD or RME enabled.

Make CTX_INCLUDE_EL2_REGS an internal macro and remove
from documentation.

Signed-off-by: Govindraj Raja <govindraj.raja@arm.com>
Change-Id: I6a70edfd88163423ff0482de094601cf794246d6
diff --git a/make_helpers/defaults.mk b/make_helpers/defaults.mk
index 683d7ac..a66123a 100644
--- a/make_helpers/defaults.mk
+++ b/make_helpers/defaults.mk
@@ -400,9 +400,10 @@
 # Enable Link Time Optimization
 ENABLE_LTO			:= 0
 
-# Build flag to include EL2 registers in cpu context save and restore during
-# S-EL2 firmware entry/exit. This flag is to be used with SPD=spmd option.
-# Default is 0.
+# This option will include EL2 registers in cpu context save and restore during
+# EL2 firmware entry/exit. Internal flag not meant for direct setting.
+# Use SPD=spmd and SPMD_SPM_AT_SEL2=1 or ENABLE_RME=1 to enable
+# CTX_INCLUDE_EL2_REGS.
 CTX_INCLUDE_EL2_REGS		:= 0
 
 # Enable Memory tag extension which is supported for architecture greater