PM / devfreq: imx-bus: use NULL to pass a null pointer rather than zero

The 3rd argument to the function of_get_property is a pointer and it is
being passed using 0. Use NULL instead.

Cleans up sparse warning:
warning: Using plain integer as NULL pointer

Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>