arm64: dts: rockchip: remove num-slots property from rk3399-sapphire

When adding the rk3399 sapphire som, two more of the recently removed
num-slots properties of dw-mmc nodes slipped in.
Remove them again.

Fixes: 8164a84c

 ("arm64: dts: rockchip: Add support for rk3399 sapphire SOM")
Signed-off-by: Heiko Stuebner <heiko@sntech.de>