ARM: rockchip: Fix use of plain integer as NULL pointer

This fixes the following sparse build warning:

     mach-rockchip/platsmp.c:68:43: Using plain integer as NULL pointer

Signed-off-by: Peter Griffin <peter.griffin@linaro.org>
Acked-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>