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/docs/getting_started/index.rst b/docs/getting_started/index.rst
index 3fbf48d..5ebabea 100644
--- a/docs/getting_started/index.rst
+++ b/docs/getting_started/index.rst
@@ -10,6 +10,7 @@
    initial-build
    tools-build
    build-options
+   build-internals
    image-terminology
    porting-guide
    psci-lib-integration-guide