ARM: dts: rockchip: clean up rk3xxx mmc nodes

Commit 356649ab

 ("ARM: dts: rockchip: unuse the slot-node and deprecate
the supports-highspeed for dw-mmc") removed the slots but not the #xx-cells
properties describing the subnodes. Do this now.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>