clocksource/drivers/timer-imx-gpt: Remove outdated file path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: NFabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: NDaniel Lezcano <daniel.lezcano@linaro.org>

drivers/clocksource/timer-imx-gpt.c

 /*
- *  linux/arch/arm/plat-mxc/time.c
- *
  *  Copyright (C) 2000-2001 Deep Blue Solutions
  *  Copyright (C) 2002 Shane Nay (shane@minirl.com)
  *  Copyright (C) 2006-2007 Pavel Pisa (ppisa@pikron.com)