ARM: OMAP: add deprecation message for legacy OMAP DMA API

The legacy OMAP DMA API is now deprecated; all remaining users should
now convert over ASAP to using the DMA engine API instead of the OMAP
private API.

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