imx: imx8mm-beacon: Enable USB booting via SDP

imx: imx8mm-beacon: Enable USB booting via SDP

In order to boot over USB, the device tree needs to enable
a few extra nodes in SPL.  Since the USB driver has the
ability to detect host/device, the dr_mode can be removed
from the device tree since it needs to act as a device when
booting and OTG is the default mode.  Add USB boot support
to spl_board_boot_device and enable the corresponding config
options.

Signed-off-by: Adam Ford <aford173@gmail.com>
Reviewed-by: Fabio Estevam <festevam@denx.de>