dt-bindings: devfreq: rk3399_dmc: Convert to YAML

I want to add, deprecate, and bugfix some properties, as well as add the
first users. This is easier with a proper schema.

The transformation is mostly straightforward, plus a few notable tweaks:

 * Renamed rockchip,dram_speed_bin to rockchip,ddr3_speed_bin. The
   driver code and the example matched, but the description was
   different. I went with the implementation. Note that this property is
   also slated for deprecation/deletion in the subsequent patches.

 * Drop upthreshold and downdifferential properties from the example.
   These were undocumented (so, wouldn't pass validation), but were
   representing software properties (governor tweaks). I drop them from
   the driver in subsequent patches.

 * Rename clock from pclk_ddr_mon to dmc_clk. The driver, DT example,
   and all downstream users matched -- the binding definition was the
   exception. Anyway, "dmc_clk" is a more appropriately generic name.

 * Choose a better filename and location (this is a memory controller).

Signed-off-by: Brian Norris <briannorris@chromium.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>