regulator: document binding for MT6315 regulator

Add device tree binding information for MT6315 regulator driver.
Example bindings for MT6315 are added.
Signed-off-by: Hsin-Hsiung Wang <hsin-hsiung.wang@mediatek.com>
Link: https://lore.kernel.org/r/1612678457-11548-2-git-send-email-hsin-hsiung.wang@mediatek.comSigned-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/regulator/mt6315-regulator.yaml

+# SPDX-License-Identifier: GPL-2.0-only OR BSD-2-Clause
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/regulator/mt6315-regulator.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Mediatek MT6315 Regulator
+
+maintainers:
+  - Hsin-Hsiung Wang <hsin-hsiung.wang@mediatek.com>
+
+description: |
+  The MT6315 is a power management IC (PMIC) configurable with SPMI.
+  that contains 4 BUCKs output which can combine with each other
+  by different efuse settings.
+
+properties:
+  compatible:
+    const: mediatek,mt6315-regulator
+
+  reg:
+    maxItems: 1
+
+  regulators:
+    type: object
+    description: List of regulators and its properties
+
+    patternProperties:
+      "^vbuck[1-4]$":
+        type: object
+        $ref: "regulator.yaml#"
+
+        properties:
+          regulator-name:
+            pattern: "^vbuck[1-4]$"
+
+    additionalProperties: false
+
+required:
+  - compatible
+  - reg
+  - regulators
+
+additionalProperties: false
+
+examples:
+  - |
+    pmic@6 {
+      compatible = "mediatek,mt6315-regulator";
+      reg = <0x6 0>;
+
+      regulators {
+        vbuck1 {
+          regulator-compatible = "vbuck1";
+          regulator-min-microvolt = <300000>;
+          regulator-max-microvolt = <1193750>;
+          regulator-enable-ramp-delay = <256>;
+          regulator-allowed-modes = <0 1 2 4>;
+        };
+
+        vbuck3 {
+          regulator-compatible = "vbuck3";
+          regulator-min-microvolt = <300000>;
+          regulator-max-microvolt = <1193750>;
+          regulator-enable-ramp-delay = <256>;
+          regulator-allowed-modes = <0 1 2 4>;
+        };
+      };
+    };