mach-ux500: fix USB build error

The mach-ux500/usb.c was referencing DMA macros, but not
including <linux/dma-mapping.h> so it didn't compile. Fixed by
a proper #include.

Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Mian Yousaf Kaukab <mian.yousaf.kaukab@stericsson.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>

arch/arm/mach-ux500/usb.c

@@ -6,6 +6,7 @@
  */
 #include <linux/platform_device.h>
 #include <linux/usb/musb.h>
+#include <linux/dma-mapping.h>
 #include <plat/ste_dma40.h>
 #include <mach/hardware.h>
 #include <mach/usb.h>