sf: support byte program for sst spi flash

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
sf: minor update for the debug output in sst_write_bp

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
sf: install sst flash write op per spi controller tx op mode

During the sst spi flash probe phase, the byte program routine
is installed by default, but if the spi controller is capable
of doing word program with auto address increment, install the
word program routine then.

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