dt-bindings: serial: document LiteUART bindings

Add documentation for LiteUART devicetree bindings.
Signed-off-by: Filip Kokosinski <fkokosinski@antmicro.com>
Signed-off-by: Mateusz Holenko <mholenko@antmicro.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Stafford Horne <shorne@gmail.com>

Documentation/devicetree/bindings/serial/litex,liteuart.yaml

+# SPDX-License-Identifier: GPL-2.0-only OR BSD-2-Clause
+
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/serial/litex,liteuart.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: LiteUART serial controller
+
+maintainers:
+  - Karol Gugala <kgugala@antmicro.com>
+  - Mateusz Holenko <mholenko@antmicro.com>
+
+description: |
+  LiteUART serial controller is a part of the LiteX FPGA SoC builder. It supports
+  multiple CPU architectures, currently including e.g. OpenRISC and RISC-V.
+
+properties:
+  compatible:
+    const: litex,liteuart
+
+  reg:
+    maxItems: 1
+
+  interrupts:
+    maxItems: 1
+
+required:
+  - compatible
+  - reg
+
+examples:
+  - |
+    uart0: serial@e0001800 {
+      compatible = "litex,liteuart";
+      reg = <0xe0001800 0x100>;
+      interrupts = <2>;
+    };