sf: Update SST25* flash params

Explicitly list supported read commands in the flash prarmas table
for SST25* flash parts. Also change SST25* flash sector size to 4KiB
to match SECT_4K. This makes 'sf erase offset +len' work on real
4KiB boundary instead of 64KiB.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>