usb/ehci-mxc: add missing inclusion of mach/hardware.h

As cpu_is_mx stuff is being used in the driver, header mach/hardware.h
should be explicitly included.

The missing of the header is causing today's linux-next build error
as bleow.

  CC      drivers/usb/host/ehci-hcd.o
In file included from linux-next/drivers/usb/host/ehci-hcd.c:1190:0:
linux-next/drivers/usb/host/ehci-mxc.c: In function 'ehci_mxc_drv_probe':
linux-next/drivers/usb/host/ehci-mxc.c:175:2: error: implicit declaration of function 'cpu_is_mx35'
linux-next/drivers/usb/host/ehci-mxc.c:175:2: error: implicit declaration of function 'cpu_is_mx25'
linux-next/drivers/usb/host/ehci-mxc.c:185:2: error: implicit declaration of function 'cpu_is_mx51'

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>