nitrogen6x: prevent warnings about board_ehci* callbacks

Include declarations of board_ehci callbacks to prevent compiler warnings
and enforce function prototypes.

Signed-off-by: Eric Nelson <eric.nelson@boundarydevices.com>

board/boundary/nitrogen6x/nitrogen6x.c

@@ -30,6 +30,7 @@
 #include <i2c.h>
 #include <input.h>
 #include <netdev.h>
+#include <usb/ehci-fsl.h>
 
 DECLARE_GLOBAL_DATA_PTR;
 #define GP_USB_OTG_PWR	IMX_GPIO_NR(3, 22)