ARM: imx: Remove epit support

Currently there is no user of EPIT, so remove such unused code.

If someone wants to add EPIT support back, then the person needs to
create a proper support into drivers/clocksource/ and add device
tree support, proper bindings, etc.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>