dt-bindings: timer: arm,twd: Convert to json-schema

Convert the ARM Timer-Watchdog Device Tree binding documentation to
json-schema.  As the old binding document actually contained two
bindings, it is split in two document: one for the timer part, and one
for the watchdog part.

Document missing properties.
Update examples to match reality.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/3ccc0cf5319f56e230ee3b8a009f8d63afb114c1.1621521847.git.geert+renesas@glider.be


[robh: Fix up node names]
Signed-off-by: Rob Herring <robh@kernel.org>