Make BL1 RO and RW base addresses configurable

BL1 RO and RW base address used to be fixed, respectively to the first
address of the Trusted ROM and the first address of the Trusted RAM.

Introduce new platform defines to configure the BL1 RO and RW base
addresses.

Change-Id: If26616513a47798593a4bb845a4b0fb37c867cd6
Make the memory layout more flexible

Currently the platform code gets to define the base address of each
boot loader image. However, the linker scripts couteract this
flexibility by enforcing a fixed overall layout of the different
images. For example, they require that the BL3-1 image sits below
the BL2 image. Choosing BL3-1 and BL2 base addresses in such a way
that it violates this constraint makes the build fail at link-time.

This patch requires the platform code to now define a limit address
for each image. The linker scripts check that the image fits within
these bounds so they don't rely anymore on the position of a given
image in regard to the others.

Fixes ARM-software/tf-issues#163

Change-Id: I8c108646825da19a6a8dfb091b613e1dd4ae133c
doc: Update information about the memory layout

Rework the "Memory layout on FVP platforms" section in the Firmware
Design document. Add information about where the TSP image fits
in the memory layout when present.

Add documentation for the base addresses of each bootloader image
in the porting guide.

Change-Id: I4afb2605e008a1cb28c44a697804f2cb6bb4c9aa