spi: dt-bindings: sifive: Add missing 2nd register region

The 'reg' description and example have a 2nd register region for memory
mapped flash, but the schema says there is only 1 region. Fix this.

Cc: Palmer Dabbelt <palmer@dabbelt.com>
Cc: linux-spi@vger.kernel.org
Cc: linux-riscv@lists.infradead.org
Acked-by: Mark Brown <broonie@kernel.org>
Acked-by: Paul Walmsley <paul.walmsley@sifive.com>
Signed-off-by: Rob Herring <robh@kernel.org>