dma: omap: hide legacy interface

The legacy interface for omap-dma is only used on OMAP1, and the
same is true for the non-DT case. Make both of these conditional on
CONFIG_ARCH_OMAP1 being set to simplify the dependency.

The non-OMAP stub functions in include/linux/omap-dma.h are note needed
any more either now, because they are only called on OMAP1.

Acked-by: Tony Lindgren <tony@atomide.com>
Acked-By: Vinod Koul <vkoul@kernel.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>