mx6sabresd: Include <usb/ehci-ci.h> header file

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

board/freescale/mx6sabresd/mx6sabresd.c:586:5: warning: no previous prototype for ‘board_ehci_hcd_init’ [-Wmissing-prototypes] int board_ehci_hcd_init(int port)

Remove this warning by including <usb/ehci-ci.h>.

Signed-off-by: Diego Dorta <diego.dorta@nxp.com>