dt-bindings: sifive: describe sifive-blocks versioning

For IP blocks that are generated from the public, open-source
sifive-blocks repository, describe the version numbering policy
that its maintainers intend to use, upon request from Rob
Herring <robh@kernel.org>.

Signed-off-by: Paul Walmsley <paul.walmsley@sifive.com>
Signed-off-by: Paul Walmsley <paul@pwsan.com>
Reviewed-by: Palmer Dabbelt <palmer@sifive.com>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Palmer Dabbelt <palmer@sifive.com>
Cc: Megan Wachs <megan@sifive.com>
Cc: Wesley Terpstra <wesley@sifive.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: devicetree@vger.kernel.org
Cc: linux-riscv@lists.infradead.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Rob Herring <robh@kernel.org>