ARM: OMAP: fix DMA vs memory ordering

Using coherent DMA memory with the OMAP DMA engine results in
unpredictable behaviour due to memory ordering issues; as things stand,
there is no guarantee that data written to coherent DMA memory will be
visible to the DMA hardware.

This is because the OMAP dma_write() accessor contains no barriers,
necessary on ARMv6 and above.  The effect of this can be seen in comments
in the OMAP serial driver, which incorrectly talks about cache flushing
for the coherent DMA stuff.

Rather than adding barriers to the accessors, add it in the DMA support
code just before we enable DMA, and just after we disable DMA.  This
avoids having barriers for every DMA register access.

Acked-by: Tony Lindgren <tony@atomide.com>
Acked-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>