ARM: imx: Add EPIT support

The Enhanced Periodic Interrupt Timer (EPIT) is found on newer
i.MX SoCs and can be used as an alternative system timer.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Michael Grzeschik <m.grzeschik@pengutronix.de>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>