soc: renesas: r9a09g056-sys: Mark rzv2n_sys_init_data as __initconst

soc: renesas: r9a09g056-sys: Mark rzv2n_sys_init_data as __initconst

Annotate rzv2n_sys_init_data with __initconst as it is only used during
initialization.

Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Claudiu Beznea <claudiu.beznea.uj@bp.renesas.com>
Link: https://patch.msgid.link/20260309165946.3003731-4-prabhakar.mahadev-lad.rj@bp.renesas.com
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>