spl: fat: Add option to disable DMA alignment

If we don't DMA-align buffers we pass to FAT, it will align them itself.
This behaviour likely should be deprecated in favor of
CONFIG_BOUNCE_BUFFER, but that's a task for another series. For the
meantime, don't bother aligning the buffer unless we had been doing so in
the past.

Signed-off-by: Sean Anderson <seanga2@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>