docs(spm): add design documentation

Add documentation how to build EL3 SPMC,
briefly describes all FF-A interfaces,
SP boot flow, SP Manifest, Power Management,
Boot Info Protocol, Runtime model and state
transition and Interrupt Handling.

Signed-off-by: Shruti Gupta <shruti.gupta@arm.com>
Change-Id: I630df1d50a4621b344a09e462563eacc90109de4
diff --git a/docs/components/index.rst b/docs/components/index.rst
index 192773f..0972a68 100644
--- a/docs/components/index.rst
+++ b/docs/components/index.rst
@@ -20,6 +20,7 @@
    romlib-design
    sdei
    secure-partition-manager
+   el3-spmc
    secure-partition-manager-mm
    xlat-tables-lib-v2-design
    cot-binding