dm: spi_flash: fix wrong dependency

dm: spi_flash: fix wrong dependency

CONFIG_SPI does not exist in Kconfig in the first place, so the
dependency "depends on DM && SPI" is never met, i.e., DM_SPI_FLASH
can never be enabled (unless you ignore the dependency in an illegal
way.  See below.)

Actually, some defconfigs such as socfpga_*_defconfig define
CONFIG_DM_SPI_FLASH=y, but it never appears in the .config file
because of this wrong dependency.

On the other hand, all the Tegra boards enable DM_SPI_FLASH because

  config DM_SPI_FLASH
          default y

silently ignores the dependency.
Unfortunately, this style of CONFIG definition is abused everywhere
in U-Boot, so we easily miss such a wrong dependency.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Simon Glass <sjg@chromium.org>