MIPS: DTS: Remove num-slots from Pistachio SoC

dwmmc driver deprecated num-slots and plan to get rid of it finally.
Just move a step to cleanup it from DT.

Cc: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>
Acked-by: James Hartley <james.hartley@sondrel.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: Jaehoon Chung <jh80.chung@samsung.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/16741/


Signed-off-by: James Hogan <jhogan@kernel.org>