ahci: flush / invalidate dcache around SATA commands

Exynos5 automatically performs DMA when the SATA controller executes
commands.  This adds the necessary dcache-to-memory flush &
invalidation calls to allow the DMA to properly function.

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