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>