spi: dt-bindings: Convert Arm pl022 to json-schema

Convert the Arm pl022 binding to DT schema format. The clock binding was
missing, so it is added to the schema. It really should be required as
well, but there are some platforms (spear) not yet using DT clock
binding.

Cc: Mark Brown <broonie@kernel.org>
Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: linux-spi@vger.kernel.org
Cc: linux-arm-kernel@lists.infradead.org
Signed-off-by: Rob Herring <robh@kernel.org>
Signed-off-by: Mark Brown <broonie@kernel.org>