dt-bindings: devfreq: rk3399_dmc: Add rockchip,pmu phandle

The Rockchip DMC (Dynamic Memory Interface) needs to access to the PMU
general register files to know the DRAM type, so add a phandle to the
syscon that manages these registers.

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