dt-bindings: timer: gpt: update binding doc

The i.MX GPT timer driver binding doc is out of date,
update it according to current GPT timer driver.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>