Document design of the Interrupt Mangement Framework ...

This patch adds documentation that describes the design of the Interrupt
management framework in the ARM Trusted Firmware. The porting-guide.md has also
been updated to describe the interface that should be implemented by each
platform to support this framework.

Change-Id: I3eda48e5c9456e6a9516956bee16a29e366633b7
Co-Authored-By: Soby Mathew <soby.mathew@arm.com>

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