zynq_cse: Turn OFF d-cache for mini u-boot

Turn OFF d-cache for zynq_cse boards, the board is
not validated properly with d-cache ON.

Signed-off-by: Jagannadha Sutradharudu Teki <jaganna@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>