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>

drivers/usb/host/ehci-xilinx-of.c

@@ -31,6 +31,7 @@
 #include <linux/of.h>
 #include <linux/of_platform.h>
 #include <linux/of_address.h>
+#include <linux/of_irq.h>
 
 /**
  * ehci_xilinx_port_handed_over - hand the port out if failed to enable it