dt-bindings: mmc: add binding for BST DWCMSHC SDHCI controller

Add device tree bindings for the Black Sesame Technologies
DWCMSHC SDHCI controller used in C1200 SoC.

The binding describes a Synopsys DesignWare Cores Mobile Storage
Host Controller with BST-specific extensions including:
- Two register regions (core SDHCI and CRM registers)
- Optional memory-region for bounce buffer support
- Fixed clock input

Signed-off-by: Ge Gordon <gordon.ge@bst.ai>
Signed-off-by: Albert Yang <yangzh0906@thundersoft.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>

Documentation/devicetree/bindings/mmc/bst,c1200-sdhci.yaml

@@ -0,0 +1,70 @@
+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/mmc/bst,c1200-sdhci.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Black Sesame Technologies DWCMSHC SDHCI Controller
+
+maintainers:
+  - Ge Gordon <gordon.ge@bst.ai>
+
+allOf:
+  - $ref: sdhci-common.yaml#
+
+properties:
+  compatible:
+    const: bst,c1200-sdhci
+
+  reg:
+    items:
+      - description: Core SDHCI registers
+      - description: CRM registers
+
+  interrupts:
+    maxItems: 1
+
+  clocks:
+    maxItems: 1
+
+  clock-names:
+    items:
+      - const: core
+
+  memory-region:
+    maxItems: 1
+
+  dma-coherent: true
+
+required:
+  - compatible
+  - reg
+  - interrupts
+  - clocks
+  - clock-names
+
+unevaluatedProperties: false
+
+examples:
+  - |
+    #include <dt-bindings/interrupt-controller/arm-gic.h>
+    #include <dt-bindings/interrupt-controller/irq.h>
+
+    bus {
+        #address-cells = <2>;
+        #size-cells = <2>;
+
+        mmc@22200000 {
+            compatible = "bst,c1200-sdhci";
+            reg = <0x0 0x22200000 0x0 0x1000>,
+                  <0x0 0x23006000 0x0 0x1000>;
+            interrupts = <GIC_SPI 144 IRQ_TYPE_LEVEL_HIGH>;
+            clocks = <&clk_mmc>;
+            clock-names = "core";
+            memory-region = <&mmc0_reserved>;
+            max-frequency = <200000000>;
+            bus-width = <8>;
+            non-removable;
+            dma-coherent;
+        };
+    };