dt-bindings: memory: tegra20: emc: Document nvidia, memory-controller property

Tegra20 External Memory Controller talks to DRAM chips and it needs to be
reprogrammed when memory frequency changes. Tegra Memory Controller sits
behind EMC and these controllers are tightly coupled. This patch adds the
new phandle property which allows to properly express connection of EMC
and MC hardware in a device-tree, it also put the Tegra20 EMC binding on
par with Tegra30+ EMC bindings, which is handy to have.
Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20201104164923.21238-6-digetx@gmail.comSigned-off-by: Krzysztof Kozlowski <krzk@kernel.org>

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

@@ -12,6 +12,7 @@ Properties:
   irrespective of ram-code configuration.
 - interrupts : Should contain EMC General interrupt.
 - clocks : Should contain EMC clock.
+- nvidia,memory-controller : Phandle of the Memory Controller node.
 
 Child device nodes describe the memory settings for different configurations and clock rates.
 
@@ -24,6 +25,7 @@ Example:
 		reg = <0x7000f400 0x400>;
 		interrupts = <0 78 0x04>;
 		clocks = <&tegra_car TEGRA20_CLK_EMC>;
+		nvidia,memory-controller = <&mc>;
 	}