spl: sdp: Add DM gadget support for sdp

When enable CONFG_SPL_DM_USB_GADGET, sdp should use
usb_gadget_initialize() and usb_gadget_release() to
support DM gadget driver.

Signed-off-by: Sherry Sun <sherry.sun@nxp.com>
Signed-off-by: Peng Fan <peng.fan@nxp.com>