spl: sdp: Add a callback to clean up USB driver

Because SDP directly jumps to next level boot image, we'd better
clean up the USB driver before it. Implement a weak callback function,
that spl sdp can use it to clean up USB driver.

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