spl: atf: move storage for bl31_params into function

There is no need to have the storage available globally. This is also a
preparation for LOAD_IMAGE_V2 support. That will introduce a similar
generator function which also has its own storage.

Signed-off-by: Michael Walle <michael@walle.cc>
Acked-by: Michal Simek <michal.simek@xilinx.com>