x86: Kconfig: Move DM_SPI & DM_SPI_FLASH to arch/Kconfig

Since all x86 boards have been converted to use DM_SPI and
DM_SPI_FLASH, move them to arch/Kconfig x86 section.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Acked-by: Simon Glass <sjg@chromium.org>