dma: Add a Kconfig for legacy DMA

dma: Add a Kconfig for legacy DMA

We cannot use the existing DMA config for the MCD driver because it is
not migrated to driver model. In order to move it to drivers/Makefile
we need some sort of option for it. Add a new DMA_LEGACY option, which
also acts as a signal that it should be migrated.

Enable this for devkit3250 which uses CONFIG_DMA_LPC32XX which is not
converted to Kconfig.

For now this is not used in the Makefile. That update happens in a
following patch.

Signed-off-by: Simon Glass <sjg@chromium.org>