sf: Move spi_flash_scan code to sf_ops

Intension is that sf_ops should deals all spi_flash
related stuff and sf_probe (which should renamed future)
should be an interface layer for spi_flash versus spi drivers.

sf_ops => spi_flash interface
sf_probe => interface layer vs spi_flash(sf_probe) to spi drivers

Tested-by: Jagan Teki <jteki@openedev.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Jagan Teki <jteki@openedev.com>