dm: sf: Save flash flags to struct spi_flash

Add a new member 'flags' in struct spi_flash to store the flash flags
during spi_flash_validate_params().

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