dt-bindings: memory: tegra20-emc: Document opp-supported-hw property

dt-bindings: memory: tegra20-emc: Document opp-supported-hw property

Document opp-supported-hw property, which is not strictly necessary to
have on Tegra20, but it's very convenient to have because all other SoC
core devices will use hardware versioning, and thus, it's good to maintain
the consistency.

Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Acked-by: Thierry Reding <treding@nvidia.com>
Link: https://lore.kernel.org/r/20201203192439.16177-2-digetx@gmail.com
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>