dt-bindings: devfreq: rk3399_dmc: Fix Hz units

dt-bindings: devfreq: rk3399_dmc: Fix Hz units

The driver and all downstream device trees [1] are using Hz units, but
the document claims MHz. DRAM frequency for these systems can't possibly
exceed 2^32-1 Hz, so the choice of unit doesn't really matter than much.

Rather than add unnecessary risk in getting the units wrong, let's just
go with the unofficial convention and make the docs match reality.

A sub-1MHz frequency is extremely unlikely, so include a minimum in the
schema, to help catch anybody who might have believed this was MHz.

[1] And notably, also those trying to upstream them:
https://lore.kernel.org/lkml/20210308233858.24741-3-daniel.lezcano@linaro.org/

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