i2c: riic: Mark riic_irqs array as const

The riic_irqs array describes the supported IRQs by the RIIC driver and
does not change at runtime.

Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Tested-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Tested-by: Claudiu Beznea <claudiu.beznea.uj@bp.renesas.com>
Reviewed-by: Claudiu Beznea <claudiu.beznea.uj@bp.renesas.com>
Reviewed-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: Andy Shevchenko <andy@kernel.org>
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>