spi: Remove CONFIG_OF_SPI_FLASH

Previous patches removed the last usages of this config variable, so
that it is now obsolete.

This patch removes it from the whitelist.

Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Jagan Teki <jagan@openedev.com>
Signed-off-by: Mario Six <mario.six@gdsys.cc>