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