dt-bindings: arm: sp810: convert to DT schema

The Arm SP810 IP is a "system controller", providing clocks, timer and a
watchdog.

Convert the DT binding to DT schema, to allow automatic validation.

The existing .txt binding described all properties as required, but the
assigned-clock* and clock-output-names are actually not (from a hardware
perspective). The only existing driver I could find (in Linux) doesn't
require them either, so drop those properties from the "required" list.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220506140533.3566431-4-andre.przywara@arm.com