dt-bindings: memory: tegra: Add external memory controller binding for Tegra210

Add the binding document for the external memory controller (EMC) which
communicates with external LPDDR4 devices. It includes the bindings of
the EMC node and a sub-node of EMC table which under the reserved memory
node. The EMC table contains the data of the rates that EMC supported.
Signed-off-by: Joseph Lo <josephl@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/memory-controllers/nvidia,tegra210-emc.yaml

+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/memory-controllers/nvidia,tegra210-emc.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: NVIDIA Tegra210 SoC External Memory Controller
+
+maintainers:
+  - Thierry Reding <thierry.reding@gmail.com>
+  - Jon Hunter <jonathanh@nvidia.com>
+
+description: |
+  The EMC interfaces with the off-chip SDRAM to service the request stream
+  sent from the memory controller.
+
+properties:
+  compatible:
+    const: nvidia,tegra210-emc
+
+  reg:
+    maxItems: 3
+
+  clocks:
+    items:
+      - description: external memory clock
+
+  clock-names:
+    items:
+      - const: emc
+
+  interrupts:
+    items:
+      - description: EMC general interrupt
+
+  memory-region:
+    $ref: /schemas/types.yaml#/definitions/phandle
+    description:
+      phandle to a reserved memory region describing the table of EMC
+      frequencies trained by the firmware
+
+  nvidia,memory-controller:
+    $ref: /schemas/types.yaml#/definitions/phandle
+    description:
+      phandle of the memory controller node
+
+required:
+  - compatible
+  - reg
+  - clocks
+  - clock-names
+  - nvidia,memory-controller
+
+additionalProperties: false
+
+examples:
+  - |
+    #include <dt-bindings/clock/tegra210-car.h>
+    #include <dt-bindings/interrupt-controller/arm-gic.h>
+
+    reserved-memory {
+        #address-cells = <1>;
+        #size-cells = <1>;
+        ranges;
+
+        emc_table: emc-table@83400000 {
+            compatible = "nvidia,tegra210-emc-table";
+            reg = <0x83400000 0x10000>;
+        };
+    };
+
+    external-memory-controller@7001b000 {
+        compatible = "nvidia,tegra210-emc";
+        reg = <0x7001b000 0x1000>,
+              <0x7001e000 0x1000>,
+              <0x7001f000 0x1000>;
+        clocks = <&tegra_car TEGRA210_CLK_EMC>;
+        clock-names = "emc";
+        interrupts = <GIC_SPI 78 IRQ_TYPE_LEVEL_HIGH>;
+        memory-region = <&emc_table>;
+        nvidia,memory-controller = <&mc>;
+    };