sf: spi_flash_validate_params => spi_flash_scan

Rename spi_flash_validate_params to spi_flash_scan
as this code not only deals with params setup but
also configure all spi_flash attributes.

And also moved all flash related code into
spi_flash_scan for future functionality addition.

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>