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>

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

@@ -14,11 +14,25 @@ Properties:
 - clocks : Should contain EMC clock.
 - nvidia,memory-controller : Phandle of the Memory Controller node.
 - #interconnect-cells : Should be 0.
+- operating-points-v2: See ../bindings/opp/opp.txt for details.
+
+Optional properties:
+- core-supply: Phandle of voltage regulator of the SoC "core" power domain.
 
 Child device nodes describe the memory settings for different configurations and clock rates.
 
 Example:
 
+	opp_table: opp-table {
+		compatible = "operating-points-v2";
+
+		opp@36000000 {
+			opp-microvolt = <950000 950000 1300000>;
+			opp-hz = /bits/ 64 <36000000>;
+		};
+		...
+	};
+
 	memory-controller@7000f400 {
 		#address-cells = < 1 >;
 		#size-cells = < 0 >;
@@ -28,6 +42,8 @@ Example:
 		interrupts = <0 78 0x04>;
 		clocks = <&tegra_car TEGRA20_CLK_EMC>;
 		nvidia,memory-controller = <&mc>;
+		core-supply = <&core_vdd_reg>;
+		operating-points-v2 = <&opp_table>;
 	}