dm: sf: Add a uclass for SPI flash

Add a driver model uclass for SPI flash which supports the common
operations (read, write, erase). Since we must keep support for the
non-dm interface, some modification of the spi_flash header is required.

CONFIG_DM_SPI_FLASH is used to enable driver model for SPI flash.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Jagannadha Sutradharudu Teki <jagannadh.teki@gmail.com>