imx: cpu: support get_boot_device for i.MX8M

Enable get_boot_device for i.MX8M, it supports boot type USB.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Fabio Estevam <fabio.estevam@nxp.com>
Reviewed-by: Stefano Babic <sbabic@denx.de>