configs: pico-imx6ul: convert DM_USB

Here is the test commands:

  => ums 0 mmc 0
  UMS: LUN 0, dev 0, hwpart 0, sector 0x0, count 0x710000

Signed-off-by: Wig Cheng <wig.cheng@technexion.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>