dt-bindings: timers: sp-804: Convert to json-schema

This converts the DT binding documentation for the ARM SP-804 timer IP
over to json-schema.
Most properties are just carried over, the clocks property requirement
(either one or three clocks) is now formalised and enforced.
As the former binding didn't specify clock-names, and there is no
common name used by the existing DTs, I refrained from adding them in
detail (just allowing the property).
The requirement for the APB clock is enforced by the primecell binding
already.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20200828142018.43298-2-andre.przywara@arm.com


[robh: drop primecell.yaml]
Signed-off-by: Rob Herring <robh@kernel.org>