spl: sdp: Add CONFIG_SPL_SDP_USB_DEV for USB device

Add a new configuration CONFIG_SPL_SDP_USB_DEV to specify the
usb index for spl sdp driver, so that we change use different device.
The default value is 0.

Signed-off-by: Ye Li <ye.li@nxp.com>
Signed-off-by: Peng Fan <peng.fan@nxp.com>