Allow BL3-2 platform definitions to be optional

The generic image loading and IO FIP code no longer forces the
platform to create BL3-2 (Secure-EL1 Payload) specific
definitions. The BL3-2 loading code in bl2/bl2main.c is wrapped
by a #ifdef BL32_BASE blocks, allowing the BL3-2 definitions to
be optional. Similarly for the name_uuid array defintion in
drivers/io/io_fip.c.

Also update the porting guide to reflect this change.

The BL3-2 platform definitions remain non-configurably present
in the FVP port.

Fixes ARM-software/tf-issues#68

Change-Id: Iea28b4e94d87a31f5522f271e290919a8a955460