spl: atf: remove helper structure from common header

bl2_to_bl31_params_mem is just an implementation detail of the SPL ATF
support and is not needed anywhere else. Move it from the header to the
actual module.

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