spi: Correct sector size to 256k for Spansion S25FL512S_256K and S25FL256S_256K

The information in sf_params.c is incorrect for the Spansion flash chips,
the suffix "256k" means that these chips have 256k sector sizes. Set the
sector size correctly and adjust the number of sectors as well.

This fixes writing to serial flash on the zynq-miami boards.

Signed-off-by: Mike Looijmans <mike.looijmans@topic.nl>
Acked-by: Siva Durga Prasad Paladugu <sivadur@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>