Add plat parameter structs to support BL2 to pass
variable-length, variable-type parameters to BL31.
The parameters are structured as a link list.
During BL31 setup time, we traverse the list to process
each parameter.

Signed-off-by: kenny liang <kenny.liang@mediatek.com>
Change-Id: Ie84cfc9606656fb1d2780a68cadf27e09afa6628