dm: ls1021aqds: dts: Use "spi_dataflash" driver instead of "spi_flash_std" for DSPI flash

The type of DSPI flash on ls1021aqds is AT45DB021, it has specail
commands and page-size.
Use the special spi flash driver instead of "spi_flash_std" driver.

Signed-off-by: Haikun Wang <haikun.wang@freescale.com>
Tested-by: Haikun Wang <haikun.wang@freescale.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Jagan Teki <jteki@openedev.com>