plat: renesas: Move headers and assembly files to common folder

Create a common directory and move the header and assembly files
so that the common code can be used by both Renesas R-Car Gen3 and
RZ/G2 platforms.

Signed-off-by: Biju Das <biju.das.jz@bp.renesas.com>
Reviewed-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Change-Id: Ia9a563a1c3c9f8c6f0d3cb82622deb2e155d7f6c
12 files changed