MLK-14312 mx7ulp: Fix incorrect DTB modification after using USB

u-boot has feature that when booting for mfgtool, the u-boot will modify the DTB
to disable SD 1.8v switch. But the judgement for mfgtool boot has a problem, it
only checks whether the USB PHY power status is enabled. When a USB device
(for example a USB ethernet) is used in u-boot, the power status is also enabled.
So the u-boot incorrectly disable the SD 1.8v switch.

The patch changes the get_boot_device to use the boot SW info provided by ROM. Only if
it is a USB boot, we will start the DTB modification for SD.

Signed-off-by: Ye Li <ye.li@nxp.com>