Convert the ARM SP-805 watchdog IP DT binding over to Json-schema.

A straight-forward conversion, but the requirement for providing two
clocks got strengthened from "should" to "must".
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Link: https://lore.kernel.org/r/20200909105046.160991-1-andre.przywara@arm.comSigned-off-by: Rob Herring <robh@kernel.org>