sf: Enable byte program support

sf: Enable byte program support

Enabled byte program support for sst flashes in sf.

Few controllers will only support BP, so this patch gives
a tx transfer flag to set the BP so-that sf will operate
on byte program transfer.

A new TX operation mode SPI_OPM_TX_BP is introduced for such SPI
controller to use byte program op for SST flash.

Signed-off-by: Jagannadha Sutradharudu Teki <jagannadh.teki@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>