dt-bindings: memory: add binding for Mediatek's MT7621 SDRAM memory controller

Add binding documentation for Mediatek's SDRAM memory controller present on
MT7621 SoC.
Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Link: https://lore.kernel.org/r/20211002060910.30613-1-sergio.paracuellos@gmail.comSigned-off-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/memory-controllers/mediatek,mt7621-memc.yaml

+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/memory-controllers/mediatek,mt7621-memc.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: MT7621 SDRAM controller
+
+maintainers:
+  - Sergio Paracuellos <sergio.paracuellos@gmail.com>
+
+properties:
+  compatible:
+    const: mediatek,mt7621-memc
+
+  reg:
+    maxItems: 1
+
+required:
+  - compatible
+  - reg
+
+additionalProperties: false
+
+examples:
+  - |
+    memory-controller@5000 {
+        compatible = "mediatek,mt7621-memc";
+        reg = <0x5000 0x1000>;
+    };