Porting Guide: Clarify identity-mapping requirement

Porting Guide: Clarify identity-mapping requirement

The memory translation library in Trusted Firmware supports
non-identity mappings for Physical to Virtual addresses since commit
f984ce84ba. However, the porting guide hasn't been updated
accordingly and still mandates the platform ports to use
identity-mapped page tables for all addresses.

This patch removes this out-dated information from the Porting Guide
and clarifies in which circumstances non-identity mapping may safely
be used.

Fixes ARM-software/tf-issues#258

Change-Id: I84dab9f3cabfc43794951b1828bfecb13049f706