USB: ehci-atmel: add needed of.h header file

Compilation error in case of non-DT configuration without this
of.h header file.
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Acked-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Acked-by: Alan Stern <stern@rowland.harvard.edu>

drivers/usb/host/ehci-atmel.c

@@ -13,6 +13,7 @@
 
 #include <linux/clk.h>
 #include <linux/platform_device.h>
+#include <linux/of.h>
 #include <linux/of_platform.h>
 
 /* interface and function clocks */