spl: atf: provide a bl2_plat_get_bl31_params_default()

Move the actual implementation of the bl2_plat_get_bl31_params() to its
own function. The weak function will just call the default
implementation. This has the advantage that board code can still call
the original implementation if it just want to modify minor things.

Signed-off-by: Michael Walle <michael@walle.cc>