spl: fix SPI config dependencies

As SPL_SPI_FLASH_SUPPORT cannot work without SPL_SPI_SUPPORT, fix
dependencies to prevent enabling SPI flash support without basic SPI
support.

Signed-off-by: Simon Goldschmidt <simon.k.r.goldschmidt@gmail.com>
Reviewed-by: Lukasz Majewski <lukma@denx.de>