sf: Fix entries for S25FL256S_256K and S25FL512S_256K

Both of these chips have 256kB big sectors, thus the _256K suffix,
compared to their _64K counterparts, which have 64kB sectors. Also,
they have four times less sectors than their _64K counterparts.

Signed-off-by: Marek Vasut <marex@denx.de>
Tested-by: Jagannadha Sutradharudu Teki <jaganna@xilinx.com>