Simplify interface to TZC-400 driver

Simplify interface to TZC-400 driver

The TZC-400 driver previously allowed the possibility of multiple
controller instances to be present in the same executable. This
was unnecessary since there will only ever be one instance.

This change simplifies the tzc_init() function to only take the
base address argument needed by implementation, conforming to the
driver initialization model of other drivers. It also hides some
of the implementation details that were previously exposed by the
API.

The FVP port has been updated accordingly.

THIS CHANGE REQUIRES ALL PLATFORM PORTS THAT USE THE TZC-400
DRIVER TO BE UPDATED

Fixes ARM-software/tf-issues#181

Change-Id: I7b721edf947064989958d8f457d6462d92e742c8