ARM: imx/clock-imx27: Pass NULL as function callback, not 0

This fixes many sparse warnings like:

	arch/arm/mach-imx/clock-imx27.c:597:1: warning: Using plain integer as NULL pointer

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