ARM i.MX23/28: platform-mxsfb: Add missing include of linux/dma-mapping.h

Fix below build error:
  CC      arch/arm/mach-mxs/devices/platform-mxsfb.o
arch/arm/mach-mxs/devices/platform-mxsfb.c: In function 'mx23_add_mxsfb':
arch/arm/mach-mxs/devices/platform-mxsfb.c:27: error: implicit declaration of function 'DMA_BIT_MASK'
make[2]: *** [arch/arm/mach-mxs/devices/platform-mxsfb.o] Error 1
make[1]: *** [arch/arm/mach-mxs/devices] Error 2
make: *** [arch/arm/mach-mxs] Error 2

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Tested-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>