clocksource/drivers/imx-gpt: Add support for ARM64

This patch allows building and compile-testing the i.MX GPT driver
also for ARM64. The delay_timer is only supported on ARMv7.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>