usb: fsl_mxc_udc.c: Fix build issue by including missing header file

Fix the following build error:

CC [M]  drivers/usb/gadget/fsl_mxc_udc.o
drivers/usb/gadget/fsl_mxc_udc.c: In function 'fsl_udc_clk_finalize':
drivers/usb/gadget/fsl_mxc_udc.c:98: error: implicit declaration of function 'readl'
drivers/usb/gadget/fsl_mxc_udc.c:100: error: implicit declaration of function 'writel'

This error is caused by the follwing commit:
(16fcb63: arm/imx: remove mx31_setup_weimcs( ) from mx31.h)

,which removed '#include <linux/io.h>' from mx31.h.

fsl_mxc_udc.c includes <mach/hardware.h>, which in turns includes mx31.h, so
that's the reason fsl_mxc_udc.c built fine previously.

Instead of relying on the indirect inclusion of the linux/io.h header file,
include it directly in fsl_mxc_udc.c.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>