dt-bindings: atmel-tcb: convert bindings to json-schema

Convert Atmel Timer Counter Blocks bindings to DT schema format using
json-schema.

Also move it out of mfd as it is not and has never been related to mfd.

Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20200710230813.1005150-2-alexandre.belloni@bootlin.com