TSP: Let the platform decide which secure memory to use

The TSP's linker script used to assume that the TSP would
execute from secure DRAM. Although it is currently the case
on FVPs, platforms are free to use any secure memory they wish.

This patch introduces the flexibility to load the TSP into any
secure memory. The platform code gets to specify the extents of
this memory in the platform header file, as well as the BL3-2 image
limit address. The latter definition allows to check in a generic way
that the BL3-2 image fits in its bounds.

Change-Id: I9450f2d8b32d74bd00b6ce57a0a1542716ab449c
fvp: Move TSP from Secure DRAM to Secure SRAM

The TSP used to execute from secure DRAM on the FVPs because there was
not enough space in Trusted SRAM to fit it in. Thanks to recent RAM
usage enhancements being implemented, we have made enough savings for
the TSP to execute in SRAM.

However, there is no contiguous free chunk of SRAM big enough to hold
the TSP. Therefore, the different bootloader images need to be moved
around to reduce memory fragmentation. This patch keeps the overall
memory layout (i.e. keeping BL1 R/W at the bottom, BL2 at the top and
BL3-1 in between) but moves the base addresses of all the bootloader
images in such a way that:
 - memory fragmentation is reduced enough to fit BL3-2 in;
 - new base addresses are suitable for release builds as well as debug
   ones;
 - each image has a few extra kilobytes for future growth.
   BL3-1 and BL3-2 are the images which received the biggest slice
   of the cake since they will most probably grow the most.

A few useful numbers for reference (valid at the time of this patch):
        |-----------------------|-------------------------------
        |  image size (debug)   |  extra space for the future
--------|-----------------------|-------------------------------
BL1 R/W |         20 KB         |            4 KB
BL2     |         44 KB         |            4 KB
BL3-1   |        108 KB         |           12 KB
BL3-2   |         56 KB         |            8 KB
--------|-----------------------|-------------------------------
Total   |        228 KB         |           28 KB       = 256 KB
--------|-----------------------|-------------------------------

Although on FVPs the TSP now executes from Trusted SRAM by default,
this patch keeps the option to execute it from Trusted DRAM. This is
controlled by the build configuration 'TSP_RAM_LOCATION'.

Fixes ARM-Software/tf-issues#81

Change-Id: Ifb9ef2befa9a2d5ac0813f7f79834df7af992b94
Doc: Add the "Building the Test Secure Payload" section

Add a section in the user guide explaining how to compile the TSP
image and include it into the FIP. This includes instructions to make
the TSP run from Trusted DRAM (rather than Trusted SRAM) on FVP.

Change-Id: I04780757a149eeb5482a12a61e821be947b882c0
Merge pull request #87 from sandrine-bailleux:sb/tf-issue-81

Conflicts:
	docs/porting-guide.md
	plat/fvp/platform.mk

Change-Id: I0e37b381161de2d42b4a66fc969a4da2fbc77a66