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>

arch/arm/mach-omap2/usb-ehci.c

@@ -20,6 +20,8 @@
 #include <linux/delay.h>
 #include <linux/platform_device.h>
 #include <linux/clk.h>
+#include <linux/dma-mapping.h>
+
 #include <asm/io.h>
 #include <plat/mux.h>