Add EL3 runtime services and SPD documentation ...

1. Add design information on EL3 runtime services and
Secure-EL1 Payload Dispatchers (SPD) to
firmware-design.md.

2. Create new EL3 runtime service writer's guide
(rt-svc-writers-guide.md) to ease creation of new
runtime services.

Change-Id: I670aeb5fc246e25c6e599a15139aac886a0074fd