dt-bindings: timer: rockchip: Add rk3588 compatible

Add compatible string for Rockchip RK3588 timer.

Signed-off-by: Cristian Ciocaltea <cristian.ciocaltea@collabora.com>
Reviewed-by: Heiko Stuebner <heiko@sntech.de>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20230419181309.338354-3-cristian.ciocaltea@collabora.com