dt-bindings: clk: add documentation for the SiFive PRCI driver

Add DT binding documentation for the Linux driver for the SiFive
PRCI clock & reset control IP block, as found on the SiFive
FU540 chip.

This version includes changes requested by Stephen Boyd
<sboyd@kernel.org> and Rob Herring <robh@kernel.org>, and
fixes some errors in the initial version.

Signed-off-by: Paul Walmsley <paul.walmsley@sifive.com>
Signed-off-by: Paul Walmsley <paul@pwsan.com>
Cc: Michael Turquette <mturquette@baylibre.com>
Cc: Stephen Boyd <sboyd@kernel.org>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Palmer Dabbelt <palmer@sifive.com>
Cc: Megan Wachs <megan@sifive.com>
Cc: linux-clk@vger.kernel.org
Cc: devicetree@vger.kernel.org
Cc: linux-riscv@lists.infradead.org
Cc: linux-kernel@vger.kernel.org
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>