MLK-20510-2 mx7d: Align implementation of is_boot_from_usb with mx6

MLK-20510-2 mx7d: Align implementation of is_boot_from_usb with mx6

Because the ROM info on iMX7D does not set device type to USB when booting
from serial download mode, we have to use the mechanism on mx6 to implement
the is_boot_from_usb. The original implementation is checking USB controller
register, it can't work correctly after any USB functionality is run in u-boot.

Signed-off-by: Ye Li <ye.li@nxp.com>
(cherry picked from commit 4934c765c6f1c9c997a28aa79f08c14fc4e91202)
(cherry picked from commit 4713c35ab51842524e4d53286dd2620c8dd9fdec)