flashchips: Enable unbounded read on W25Q256 Mark unbounded read as supported and tested as I verified it on a W25Q256 chip. BUG=b:117318564 BRANCH=none TEST=test read on W25Q256 Change-Id: I6f1cf505b5cc9bef8930868e566a7fc62034889e Signed-off-by: Duncan Laurie <dlaurie@google.com> Reviewed-on: https://chromium-review.googlesource.com/1273627 Reviewed-by: Simon Glass <sjg@chromium.org> Reviewed-by: Furquan Shaikh <furquan@chromium.org>