board: sl28: enable USB periheral support and gadgets

Enable support to update the board via the DFU protocol and make it
possible to export the block devices via USB mass storage protocol.

This will not work out of the box, yet. You have to change the dr_mode
of the usb0 controller to peripheral manually to make it work. True, OTG
support will hopefully coming soon.

Signed-off-by: Michael Walle <michael@walle.cc>
Reviewed-by: Priyanka Jain <priyanka.jain@nxp.com>