dt-bindings: timer: fsl,imxgpt: Add i.MX8MP variant

The i.MX8MP has the same register layout as the i.MX6DL, so add it as a
variant allowing to add the GPT IP blocks to the i.MX8MP's dtsi file.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20230327173526.851734-2-u.kleine-koenig@pengutronix.de


Signed-off-by: Rob Herring <robh@kernel.org>