ARM: imx6: dh-imx6: Drop the SF hunk in config

ARM: imx6: dh-imx6: Drop the SF hunk in config

There is now CONFIG_SPL_DM_SPI{,_FLASH}, however keeping
CONFIG_DM_SPI{,_FLASH} enabled in SPL seems to grow the SPL
by a couple of bytes:
      text    data     bss     dec     hex filename
 -   34069    1568      96   35733    8b95 spl/u-boot-spl
 +   34075    1568      96   35739    8b9b spl/u-boot-spl
In either case, the binary is bootable, so remove the part
in board config.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Andreas Geisreiter <ageisreiter@dh-electronics.de>
Cc: Jagan Teki <jagan@amarulasolutions.com>
Cc: Ludwig Zenz <lzenz@dh-electronics.de>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Tom Rini <trini@konsulko.com>