clk: imx6: add EPIT clock support

Add EPIT clock support to the i.MX6Q clocking infrastructure.

Signed-off-by: Colin Didier <colin.didier@devialet.com>
Signed-off-by: Clément Peron <clement.peron@devialet.com>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>