clocksource/drivers/timer-imx-gpt: Remove outdated file path

imx gpt timer driver has been moved from arch/arm/mach-imx to
drivers/clocksource/ a long time ago, so remove the reference
to the old file path.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>