dt-bindings: devfreq: rk3399_dmc: move interrupts to be optional.

In ATF we already wait for DDR DVFS finish, so move the interrupts properties
to be optional.

Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Reviewed-by: Chanwoo Choi <cw00.choi@samsung.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: MyungJoo Ham <myungjoo.ham@samsung.com>