dt-bindings: soc: microchip: document the simple-mfd syscon on PolarFire SoC

"mss-top-sysreg" contains clocks, pinctrl, resets, an interrupt controller
and more. At this point, only the reset controller child is described as
that's all that is described by the existing bindings.
The clock controller already has a dedicated node, and will retain it as
there are other clock regions, so like the mailbox, a compatible-based
lookup of the syscon is sufficient to keep the clock driver working as
before, so no child is needed. There's also an interrupt multiplexing
service provided by this syscon, for which there is work in progress at
[1].

Link: https://lore.kernel.org/linux-gpio/20240723-uncouple-enforcer-7c48e4a4fefe@wendy/ [1]
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>

Documentation/devicetree/bindings/soc/microchip/microchip,mpfs-mss-top-sysreg.yaml

@@ -0,0 +1,47 @@
+# SPDX-License-Identifier: GPL-2.0-only OR BSD-2-Clause
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/soc/microchip/microchip,mpfs-mss-top-sysreg.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Microchip PolarFire SoC Microprocessor Subsystem (MSS) sysreg register region
+
+maintainers:
+  - Conor Dooley <conor.dooley@microchip.com>
+
+description:
+  An wide assortment of registers that control elements of the MSS on PolarFire
+  SoC, including pinmuxing, resets and clocks among others.
+
+properties:
+  compatible:
+    items:
+      - const: microchip,mpfs-mss-top-sysreg
+      - const: syscon
+
+  reg:
+    maxItems: 1
+
+  '#reset-cells':
+    description:
+      The AHB/AXI peripherals on the PolarFire SoC have reset support, so
+      from CLK_ENVM to CLK_CFM. The reset consumer should specify the
+      desired peripheral via the clock ID in its "resets" phandle cell.
+      See include/dt-bindings/clock/microchip,mpfs-clock.h for the full list
+      of PolarFire clock/reset IDs.
+    const: 1
+
+required:
+  - compatible
+  - reg
+
+additionalProperties: false
+
+examples:
+  - |
+    syscon@20002000 {
+      compatible = "microchip,mpfs-mss-top-sysreg", "syscon";
+      reg = <0x20002000 0x1000>;
+      #reset-cells = <1>;
+    };
+