imx:mx6slevk add board level support for usb

imx:mx6slevk add board level support for usb

Add pinmux settings, implement board_ehci_hcd_init, board_usb_phy_mode

There are two usb port on mx6slevk board:
1. otg port
2. host port
The following are the connection between usb controller and board usb
interface, host port has not ID pin set:
otg1 core <---> board otg port
otg2 core <---> board host port
In order to make host port work, board_usb_phy_mode return USB_INIT_HOST
to let host port work in host mode.

Signed-off-by: Peng Fan <Peng.Fan@freescale.com>
Signed-off-by: Ye Li <B37916@freescale.com>