This platform supports multiple SoCs. The next SoC will still keep
quite similar architecture, but the memory base will be changed.

The ENABLE_PIE improves the maintainability and usability. You can reuse
a single set of BL images for other SoC/board without re-compiling TF-A
at all. This will also keep the code cleaner because it avoids #ifdef
around various base addresses.

By defining ENABLE_PIE, BL2_AT_EL3, BL31, and BL32 (TSP) are really
position-independent now. You can load them anywhere irrespective of
their link address.

Change-Id: I8d5e3124ee30012f5b3bfa278b0baff8efd2fff7
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>