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: Diego Dorta <diego.dorta@nxp.com>