Revert "spi: xilinx: Use standard num-cs binding"

This reverts commit 6265822a.

Based on my mainline discussion with Mark Brown here
https://lkml.org/lkml/2015/3/31/214


num-cs is software property from his point of view and
it shouldn't be used at all.
That's why origin patch is reverted.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>