ARM: omap1: move plat/dma.c to mach/omap-dma.c

Most of the interface functions in plat/dma.c are only used from the
USB driver, which is practically OMAP1 specific, except for compile
testing.

The omap_get_plat_info(), omap_request_dma() and omap_free_dma()
functions are never called on omap2 because of runtime checks.

Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>