omap: fix build failure due to missing include dma-mapping.h

Fixes following error,

CC      arch/arm/mach-omap2/usb-ehci.o
arch/arm/mach-omap2/usb-ehci.c:263: error: implicit declaration of function
'DMA_BIT_MASK'
arch/arm/mach-omap2/usb-ehci.c:263: error: initializer element is not constant
make[1]: *** [arch/arm/mach-omap2/usb-ehci.o] Error 1

Signed-off-by: Amit Kucheria <amit.kucheria@canonical.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>