pwm: ep93xx: Prepare clock before using it

Use clk_prepare_enable()/clk_disable_unprepare() in preparation for switch
to Common Clock Framework.

Signed-off-by: Alexander Sverdlin <alexander.sverdlin@gmail.com>
Reviewed-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>