imx: mx6: fix USB bmode to use reserved value

Currently the bmode "usb" uses BOOT_CFG1 to 0x01, -which means
BOOT_CFG1[7:4] is set to b0000. According to Table 8-7 Boot
Device Selection this is NOR/OneNAND and not Reserved.

Use 0x10 which leads to b0001, which is a Reserved boot device.
With that the SoC reliably falls back to the serial loader.

Cc: Troy Kisky <troy.kisky@boundarydevices.com>
Signed-off-by: Stefan Agner <stefan.agner@toradex.com>
Tested-by: Troy Kisky <troy.kisky@boundarydevices.com>