arm64: dts: rockchip: remove num-slots from all platforms

We deprecated the "num-slots" property now and plan to get
rid of it finally. Just move a step to cleanup it from DT.

Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>