dt-bindings: memory: tegra20: emc: Document OPP table and voltage regulator

dt-bindings: memory: tegra20: emc: Document OPP table and voltage regulator

The SoC core voltage can't be changed without taking into account the
clock rate of External Memory Controller. Document OPP table that will
be used for dynamic voltage frequency scaling, taking into account EMC
voltage requirement. Document optional core voltage regulator, which is
optional because some boards may have a fixed core regulator and still
frequency scaling may be desired to have.

Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20201104164923.21238-9-digetx@gmail.com
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>