ARM: dts: zte: remove num-slots from zx296702-ad1

dwmmc driver deprecated num-slots 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: Jun Nie <jun.nie@linaro.org>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>