MAINTAINERS: Update drm/rcar-du maintainers

Update drm/rcar-du maintainer entries:

* Add myself as drm/rcar-du maintainer.
* Update Laurent's email to include +renesas.
* Switch Kieran from a maintainer to reviewer.
* Change rcar-du to be under drm-misc umbrella.

Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Acked-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Acked-by: Maxime Ripard <mripard@kernel.org>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250128-rcar-du-maintainers-v2-1-4a3860a3e1ef@ideasonboard.com