dt-bindings: memory: tegra30: emc: Document OPP table and voltage regulator

dt-bindings: memory: tegra30: emc: Document OPP table and voltage regulator

Document new OPP table and voltage regulator properties which are needed
for supporting dynamic voltage-frequency scaling of the memory controller.
Some boards may have a fixed core voltage regulator, hence it's optional
because frequency scaling still may be desired.

Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20201104164923.21238-12-digetx@gmail.com
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>