arm: fix implicit use of hardware.h in ehci-mxc.c

arm: fix implicit use of hardware.h in ehci-mxc.c

To fix this:

drivers/usb/host/ehci-mxc.c: In function 'ehci_mxc_drv_probe':
drivers/usb/host/ehci-mxc.c:175: error: implicit declaration of function 'cpu_is_mx35'
drivers/usb/host/ehci-mxc.c:175: error: implicit declaration of function 'cpu_is_mx25'
drivers/usb/host/ehci-mxc.c:185: error: implicit declaration of function 'cpu_is_mx51'
make[5]: *** [drivers/usb/host/ehci-hcd.o] Error 1

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>