pwm: jz4740: Force dependency on Device Tree

Ingenic SoCs all require CONFIG_OF, so there is no case where we want to
use this driver without CONFIG_OF.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>