clocksource/drivers/imx-gpt: Fold <soc/imx/timer.h> into its only user

Only the imx-gpt timer driver makes use of enum imx_gpt_type that is
otherwise unused. Move its definition into the timer-imx-gpt driver.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20230328100531.879485-3-u.kleine-koenig@pengutronix.de