arm: mach-rmobile: Mark the default s_init function as weak

Mark the default s_init function as weak, so that SoC's can
override it if needed, and it will still be discarded if unused.

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