drivers/usb/host/ehci-xilinx-of.c: Include "linux/of_irq.h" to avoid compiling error

drivers/usb/host/ehci-xilinx-of.c: Include "linux/of_irq.h" to avoid compiling error

Need include it for irq_of_parse_and_map(), the related error with
allmodconfig under microblaze:

  drivers/usb/host/ehci-xilinx-of.c: In function ‘ehci_hcd_xilinx_of_probe’:
  drivers/usb/host/ehci-xilinx-of.c:156:2: error: implicit declaration of function ‘irq_of_parse_and_map’ [-Werror=implicit-function-declaration]
    irq = irq_of_parse_and_map(dn, 0);
    ^

Signed-off-by: Chen Gang <gang.chen.5i5j@gmail.com>
Acked-by: Michal Simek <monstr@monstr.eu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>