clocksource/drivers/timer-imx-gpt: Remove non-DT function

mxc_timer_init() was originally only used by non-DT i.MX platforms.

i.MX has already been converted to be a DT-only platform.

Remove the unused mxc_timer_init() function.

Signed-off-by: Fabio Estevam <festevam@denx.de>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20230307124313.708255-1-festevam@denx.de