dt-bindings: riscv: sifive: add YAML documentation for the SiFive FU540

Add YAML DT binding documentation for the SiFive FU540 SoC.  This
SoC is documented at:

    https://static.dev.sifive.com/FU540-C000-v1.0.pdf

Passes dt-doc-validate, as of yaml-bindings commit 4c79d42e9216.
Signed-off-by: Paul Walmsley <paul.walmsley@sifive.com>
Signed-off-by: Paul Walmsley <paul@pwsan.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Palmer Dabbelt <palmer@sifive.com>
Cc: Albert Ou <aou@eecs.berkeley.edu>
Cc: devicetree@vger.kernel.org
Cc: linux-riscv@lists.infradead.org
Cc: linux-kernel@vger.kernel.org

Documentation/devicetree/bindings/riscv/sifive.yaml

+# SPDX-License-Identifier: (GPL-2.0 OR MIT)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/riscv/sifive.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: SiFive SoC-based boards
+
+maintainers:
+  - Paul Walmsley <paul.walmsley@sifive.com>
+  - Palmer Dabbelt <palmer@sifive.com>
+
+description:
+  SiFive SoC-based boards
+
+properties:
+  $nodename:
+    const: '/'
+  compatible:
+    items:
+      - enum:
+          - sifive,freedom-unleashed-a00
+      - const: sifive,fu540-c000
+      - const: sifive,fu540
+...

MAINTAINERS

@@ -14322,6 +14322,15 @@ S:	Supported
 K:	sifive
 N:	sifive
 
+SIFIVE FU540 SYSTEM-ON-CHIP
+M:	Paul Walmsley <paul.walmsley@sifive.com>
+M:	Palmer Dabbelt <palmer@sifive.com>
+L:	linux-riscv@lists.infradead.org
+T:	git git://git.kernel.org/pub/scm/linux/kernel/git/pjw/sifive.git
+S:	Supported
+K:	fu540
+N:	fu540
+
 SILEAD TOUCHSCREEN DRIVER
 M:	Hans de Goede <hdegoede@redhat.com>
 L:	linux-input@vger.kernel.org