ARM: omap: dma: make usb support optional

Most of the plat-omap/dma.c code is specific to the USB
driver. Hide that code when it is not in use, to make it
clearer which parts are actually still required.

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