sf: probe: Add support for SST_WP

Most of the SST flashes needs to write up using SST_WP, AAI
Word Program, so added a flag param on spi_flash_params table.

SST flashes, which supports SST_WP need to use a WP write
sst_write_wp instead of common flash write.

Signed-off-by: Jagannadha Sutradharudu Teki <jaganna@xilinx.com>