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.comSigned-off-by: Krzysztof Kozlowski <krzk@kernel.org>

Documentation/devicetree/bindings/memory-controllers/nvidia,tegra20-emc.txt

@@ -16,6 +16,12 @@ Properties:
 - #interconnect-cells : Should be 0.
 - operating-points-v2: See ../bindings/opp/opp.txt for details.
 
+For each opp entry in 'operating-points-v2' table:
+- opp-supported-hw: One bitfield indicating SoC process ID mask
+
+	A bitwise AND is performed against this value and if any bit
+	matches, the OPP gets enabled.
+
 Optional properties:
 - core-supply: Phandle of voltage regulator of the SoC "core" power domain.