Now that we've changed the driver to require a chipselect GPIO, let's update the devicetree binding document. BUG=chrome-os-partner:35348 TEST=SPI NAND test on BuB. Change-Id: Ib0d2585b2d7c9ec7a5ad0c9150befe998ea3728e Signed-off-by: Ezequiel Garcia <ezequiel.garcia@imgtec.com> Reviewed-on: https://chromium-review.googlesource.com/261683 Reviewed-by: Sifan Naeem <sifan.naeem@imgtec.com> Reviewed-by: Andrew Bresticker <abrestic@chromium.org> Tested-by: Andrew Bresticker <abrestic@chromium.org> Commit-Queue: Andrew Bresticker <abrestic@chromium.org>