dt-bindings: mfd: ti,am3359-tscadc: Add a yaml description for this MFD

There is a very light description of this MFD in a text file dedicated
to a touchscreen controller (which is one of the two children of the
MFD). Here is now a complete yaml description.
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20211015081506.933180-3-miquel.raynal@bootlin.com

Documentation/devicetree/bindings/mfd/ti,am3359-tscadc.yaml

+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/mfd/ti,am3359-tscadc.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: TI AM3359 Touchscreen controller/ADC
+
+maintainers:
+  - Miquel Raynal <miquel.raynal@bootlin.com>
+
+properties:
+  compatible:
+    oneOf:
+      - const: ti,am3359-tscadc
+      - items:
+          - const: ti,am654-tscadc
+          - const: ti,am3359-tscadc
+
+  reg:
+    maxItems: 1
+
+  interrupts:
+    maxItems: 1
+
+  clocks:
+    maxItems: 1
+
+  clock-names:
+    const: fck
+
+  dmas:
+    items:
+      - description: DMA controller phandle and request line for FIFO0
+      - description: DMA controller phandle and request line for FIFO1
+
+  dma-names:
+    items:
+      - const: fifo0
+      - const: fifo1
+
+  adc:
+    type: object
+    description: ADC child
+
+  tsc:
+    type: object
+    description: Touchscreen controller child
+
+required:
+  - compatible
+  - reg
+  - interrupts
+  - clocks
+  - clock-names
+  - dmas
+  - dma-names
+
+additionalProperties: false
+
+examples:
+  - |
+    #include <dt-bindings/interrupt-controller/arm-gic.h>
+
+    tscadc@0 {
+        compatible = "ti,am3359-tscadc";
+        reg = <0x0 0x1000>;
+        interrupts = <GIC_SPI 16 IRQ_TYPE_LEVEL_HIGH>;
+        clocks = <&adc_tsc_fck>;
+        clock-names = "fck";
+        dmas = <&edma 53 0>, <&edma 57 0>;
+        dma-names = "fifo0", "fifo1";
+
+        tsc {
+        };
+
+        adc {
+        };
+    };