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/Makefile b/Makefile
index 1ddb7b8..c4350dc 100644
--- a/Makefile
+++ b/Makefile
@@ -526,9 +526,7 @@
         SPD_DIR := std_svc
 
         ifeq ($(SPMD_SPM_AT_SEL2),1)
-            ifeq ($(CTX_INCLUDE_EL2_REGS),0)
-                $(error SPMD with SPM at S-EL2 requires CTX_INCLUDE_EL2_REGS option)
-            endif
+            CTX_INCLUDE_EL2_REGS := 1
 	    ifeq ($(SPMC_AT_EL3),1)
                 $(error SPM cannot be enabled in both S-EL2 and EL3.)
             endif
@@ -574,6 +572,14 @@
     # over the sources.
 endif
 
+ifeq (${CTX_INCLUDE_EL2_REGS}, 1)
+ifeq (${SPD},none)
+ifeq (${ENABLE_RME},0)
+    $(error CTX_INCLUDE_EL2_REGS is available only when SPD or RME is enabled)
+endif
+endif
+endif
+
 ################################################################################
 # Include rmmd Makefile if RME is enabled
 ################################################################################