[U-Boot] Remove atmel_dataflash_spi.c

Hi Stelian/Alison,

Need some inputs here, since atmel dataflash driver is already up-streamed on flash side at drivers/mtd/spi/sf_dataflash.c then what is the point of using drivers/spi/atmel_dataflash_spi.c at spi side.

Are they identically same? can't we use sf_dataflash for common dataflash operations like what we does in atmel_dataflash_spi?

Thanks!

Jagan.