spi: Move DM_SPI_FLASH and SPI_FLASH_DATAFLASH to Kconfig (for ls1021aXXX)

This patch moves the CONFIG_DM_SPI_FLASH and CONFIG_SPI_FLASH_DATAFLASH
to be defined in Kconfig, not in board specific header file
(include/configs/<board>.h).

Before this change the CONFIG_DM_SPI_FLASH was not set in .config (so it
was not possible to use CONFIG_IS_ENABLED(DM_SPI_FLASH) in SPI DM/DTS
converted drivers), but it was set in u-boot.cfg file.

Signed-off-by: Lukasz Majewski <lukma@denx.de>
Signed-off-by: Hou Zhiqiang <Zhiqiang.Hou@nxp.com>