Clarify platform porting interface to TSP

Clarify platform porting interface to TSP

* Move TSP platform porting functions to new file:
  include/bl32/tsp/platform_tsp.h.

* Create new TSP_IRQ_SEC_PHY_TIMER definition for use by the generic
  TSP interrupt handling code, instead of depending on the FVP
  specific definition IRQ_SEC_PHY_TIMER.

* Rename TSP platform porting functions from bl32_* to tsp_*, and
  definitions from BL32_* to TSP_*.

* Update generic TSP code to use new platform porting function names
  and definitions.

* Update FVP port accordingly and move all TSP source files to:
  plat/fvp/tsp/.

* Update porting guide with above changes.

Note: THIS CHANGE REQUIRES ALL PLATFORM PORTS OF THE TSP TO
      BE UPDATED

Fixes ARM-software/tf-issues#167

Change-Id: Ic0ff8caf72aebb378d378193d2f017599fc6b78f