dt-bindings: devfreq: rk3399_dmc: Remove references of unexistant defines

Those DDR related defines do not exist. Replace their references with
their numerical constant.

Signed-off-by: Gaël PORTAY <gael.portay@collabora.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>