usb: ehci-ci: Add a prototype for board_ehci_power()

When compiling with W=1 the following warning is observed:

board/freescale/mx6sabresd/mx6sabresd.c:601:5: warning: no previous prototype for ‘board_ehci_power’ [-Wmissing-prototypes] int board_ehci_power(int port, int on)

Remove this warning by adding the function prototype into usb/ehci-ci.h file.
Signed-off-by: NDiego Dorta <diego.dorta@nxp.com>

include/usb/ehci-ci.h

@@ -280,6 +280,7 @@ struct usb_ehci {
 int usb_phy_mode(int port);
 /* Board-specific initialization */
 int board_ehci_hcd_init(int port);
+int board_ehci_power(int port, int on);
 int board_usb_phy_mode(int port);
 
 #endif /* _EHCI_CI_H */