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