rcar: Change SCP activation to notify TF-A using SDS

SCP boot has actually a fixed wait time of 1000ms.
This patch uses SDS in order to improve the synchronization delay on boot.

Signed-off-by: Tsutomu Muroya <tsutomu.muroya.jy@bp.renesas.com>
Signed-off-by: Nicolas Royer <nroyer@baylibre.com>